M.L.
Don't correct your son - you'll probably make him self-conscious. Just take your camera with you every time you can. You have to catch him unaware!
I know because I have grandchildren who do this. The good thing is that when a shot is needed, I'll always get smiles (and they're nice smiles). But it takes some doing to get the candids. However, that's a lot of fun - and when you need a predictable smile for the relatives, you know you'll get that, too.
So think of your photography as being documentary photography from now on, and go from there.
Not too bad a problem to have!
(A zoom lens is good. You'll have fun with it anyhow.)