I think you should. As PP have said, in these days of genetic ancestry testing and that sort of stuff, the chances that he will find out are quite high. And you've told your parents, so you're now in the situation where you'd be keeping secret from him something immensely important to him, while having told other people, which I think is unfair (possibly my view here is coloured by the fact that one of my friends is doing just this to her daughter - having told several of her friends that she had to use donor sperm; I hate the idea that I know stuff about her daughter's parentage that her daughter doesn't.)
Basically you have two choices.
One. Tell him right from the start in an age appropriate way - friends of mine have gone for "to make a baby you need mummy ingredients and daddy ingredients - and daddy didn't make very good ingredients, so a nice doctor helped them use another man's baby ingredients to make you. But a daddy is the person who loves you and cuddles you and tucks you up in bed at night and reads you stories - and daddy does all that for you." That way there's no huge moment where the big secret is revealed, it's just part of his story from the very beginning.
Or two: hide it, he finds out (blood test, DNA ancestry kit, whatever happenstance) and feels incredibly hurt and betrayed and it really screws up your relationship with him as adults.
I know which I'd choose.
(Further disclosure - my parent hid a big family secret from me - not this precise one, but of similar magnitude. The knock-on lies they had to tell to keep it from me left some big elephant sized unexplained holes in my childhood story, which - because this is what children do - I tried to fill from my imagination. I created worst-case explanations of what it was they were hiding, which really screwed up my relationship with my mum through my teens. Trust me, you do not want to go down this path, it will not end well.)