Just for few days... we have noticed the neck of our dog starting to enlarge and swell... this is actually awkward since our dog doesn't mind of we touch it.. no pain... and also his appetite is normal and he did become very active...
What does this mean...
And what should we do..

It could be a lot of things - he could have been bitten by an insect such as a tic or wasp and may be reacting to that. My dog has twice been stung by wasps on her snout and it swelled up considerably both times - I did take her to the vet and they prescribed treatment her for an allergic reaction.

He also could have some sort of infection that's causing his lymph glands to swell. There are a lot of glands in the neck area, so it can be hard to say - maybe could be a thyroid issue, too.

In any event, it's not normal so you should take him to the vet immediately - I don't think anyone can really diagnose over the internet, even if there's a great vet on here - they'd tell you to take the dog in and have him checked out.