Im not a mom so i dont have too much experience but im 20 so I might be able to help. ask yourself these questions
D I think its important to tell him because I want to be honest and I feel that if I dont tell him I am essentially lying?
Do I think he aught to know just because the whole family is aware of it?
Is it possible he knows but never brings it up? This seems pretty typical of 1) a 21 year old guy. Another likely situation is that he doesnt know, and doesnt care to know because hes dealing with his own life
Maybe you can bring it up casually somehow. I am not sure.