My rotte is looks very slim what I can feed to gain weight?

How old is your Rottie?

Can you post any photos (side on and standing over) to give us an idea of the dogs weight?

A picture would help a lot.

What does your vet say about your dogs weight? Is he up to date with worm and flea treatment? What type of food are you feeding? If you can provide us with a little more info, we might be able to give you a better idea of what to expect for his age.