My hubby Brad and son Tom aged 6 took Oscar (Shih Tzu) to the park today after playing off lead Tom went to put Oscar back on his lead and Oscar snapped at Tom just catching his chin. Tom is really upset about it and thinks Oscar doesn't like him anymore etc etc, Oscar is 20 weeks now is he starting to assert himself because he has always been very laid back and tolerant with the kids.

How should I treat this if it happens again?

I've told Tom he is to stop picking Oscar up because I sense Oscar isn't particularly happy with it anymore.

Any advice would be greatly appreciated.

To be honest he should never have been picking him up, as you say as he gets older he won't like it. 

Dogs will put up with being picked up and some may like their owners that they trust doing it, but children is another matter. 

They haven't the same status or entitlements in the dogs eyes to the adults in the household.  They can also pester or hurt the dog inadvertently.

You should train him to come when called and sit nicely to ahve his lead put on, with a treat for doing it nicely.  You could then progress to doing thid but have your don give the treat, and then wehn he is hppy then progress to letting yoru son attach the lead, as he will know the treat will follow so he will tolerate the perceived indignity.  This should all be done on the ground.

You could supervise very short training sessions between the two, this will build trust and a better relationship between them of mutual respect.

children can be a little rough with dogs and puppies alike  without meaning too.... maybe he accidently pulled his coat while putting on the lead .
He also probably didnt want his lead on as he knew he would be going  home and fun was over for the day.
I think it best that you dont let  children handle the dog ....you hold him while still on the floor and let your son gently stroke him
why not join a training class and take your son along to learn how to behave around a dog/puppy (not saying that he doesnt already)to build up his confidence around dogs/puppies again with the advice of a trained instructor and you just might enjoy it too.

there will probably be someone along soon with more knowledge on the subject than myself.

good luck

edited to add: see..... brainless your a quick one