Help! - Rough play

My dog Lincoln will be three months old on the 6th. He is growing into his big boy body (part great dane) and I'm trying to correct any bad behavior before he gets TOO big. he is doing very well with basic commands like sit, shake and "be nice" which I say when rewarding behavior with treats and that simply means to not snap at my hand and to be gentle. but I'm having a hard time trying to enforce the gentleness with my 5-year-old son. When he plays with my son I notice he is a lot more vocal, or even when my son is petting him he does a playful growl and likes to bite on his fingers all which he does not do with me. Im sure he can sense that my son is younger and his vet said that they will fight for dominance over children and that it is my job to let him now that my son is not his equal but in a nice way. Can anybody recommend any tips? I would also like to throw out there that my son is pretty gentle with him and has been since we brought him home but is kind of starting to avoid him now being that Lincoln is always rough when playing with him.