We rescued a 20 mnth old blk lab mix a little over a mnth ago. He is very skinny. We feed him in the a.m. And p.m. And a less amnt through out the day. We also give him rice mixed in with his food. He has A lot of energy and loves to play fetch. He was chained up for a number of months with no human contact for mnths so we talk him on walks A lot. He is hardly ever chained up. My question or questions actually: what can we feed him to help him gain weight and should we limit fetch and walks?

What are you feeding him now? A good quality dry food should be adequate but keep in mind that good food is not cheap steer clear of the Ol Roy and Alpo type foods. Depending on what you are able to spend the higher quality Purina brands like Pro Plan, or Natural Choice, Blue Buffalo, the list goes on and on. You may also want to think about mixing in a quality puppy food for the added protein and calories. Again, many choices, there are many quality puppy foods out there but I have always had good luck with Science Diet Growth. The bags of food themselves should have feeding guidelines on them. Don't resort to fatty table foods they can cause more problems than a few.
As far as exercise goes he sounds like he is pretty high-energy and active, he will need exercise to lengthen and build muscle mass.
Keep in mind also that some breeds are naturally thin, so that will also play a part.
Last and certainly not least, he needs a good check-up and vaccines if you haven't done this already, while there he can be checked for intestinal parasites that may be hindering his growth.

We give him pro plan mixed with human rice. I just read last night about puppy food.. do we mix that with the adult dog food? He has his vaccinations. And we are bringing him to the vet this week to get fixed and will have them check for other things as well. Like I said we've had him for a little over a month. He's aggressive and has biten us a number of times so we have been bringing him to training before the vet. We were unsure if we would be able to keep him because he bit a child. And also bit my face requiring stitches. But he has come along way and he's trying so hard to be a good boy. Unfortunately we have been ordered to keep him in his room while boyfriend kids are here so its going to take longer for him to trust
Them. So.. anyway back to the weight issue.. I read last night about an abused dog possibly having neurological issues and that's why he's not gaining. Now my dog was not as abused as that particular dog but how do you KNOW if its neurological?? He seems happy now that he has started to trust us and has learned that WE are good humans that just want to love him. But is having him separated from family every other wknd setting him back.? We plan to bring the kids to training . And for now hoping they see how much he has changed. He is such a good boy. :).. . Thanks for your advice! We will try puppy food! :)

Aggression opens up a whole new kettle of fish. I don't know that having him separated is holding him back but if it keeps the kids safe then it seems to me a necessary evil. When it comes to biting dogs and kids I tend to sit on the kids side of the table, even more so if the biting is unprovoked. Don't misunderstand me when I say that not only does your new dog need to trust you, you need to able to trust your new dog. The bottom line there is that any injury or damage to human or another animal that your pet causes is your liability. However, you seem to have taken responsible steps so far and with any luck neutering and training will help with those issues.
That being said-back to the food.
An abrupt change will likely cause stomach upset, it will be best to simply supplement his normal meal with about 1/4 cup of puppy food at each feeding. Doesn't sound like much but puppy foods are high in calories and it may be that he only needs a boost.
Neutering should also help with weight gain.
Check out this recent thread, pay particular notice to the weight chart posted by one of the members.
It is a really good diagram of where your dog stands weight wise.

https://www.askmehelpdesk.com/dogs/adult-female-boxer-too-thin-629099.html

Maybe someone more intune to behavioral issues than myself will chime in and be able to give you some tips there.
I wish you the best with your new guy

Thank you ladysam! We will try that with the food. And hope that the neutering helps as well. And I do agree with keeping him separated from the kids. We just hope that one day we can all live happy free and fearless in the same house. He so badly wants to be a part of the bigger family. He has come a long way in a month. Its been a bit overwhelming for him I'm sure. Just going from the big outdoors with nobody to being in a house with people. But I know he's enjoying the warmth of the home and human affection. :) thanks for your help.

I hope it helps and I really do hope his biting resolves for your sake, you seem to be completely in love with him and I applaud your efforts.

Its been 3 weeks since he has biten or even growled.. We can tell he's trying. And yes we love him lots! All dogs deserve love. Thank you so much!