My kids love the "I forget" infact, I loved "I forget" all through my growing up years. It meant I didn't have to try to remember, if I thought I was going to be in trouble for something if I "Forgot" then maybe the punishment would be less (which it never was, but kids, have their own thought patterns). My 3 year old says I forget, often when asked a question about what we did or what something is. But with gentle prodding she'll announce the correct or close to correct answer.
If when your son says I forget, you tell him the answer, you're re-affirming the behaviour and making it easy for him. He'll quickly figure out that by "not knowing" Mommy will do the work, easy! If after you've covered a letter, color etc. you ask him a question that you know he knows the answer to and are met with the "I forget" response, try saying, "Oh, I know it's sometimes hard to remember new things but I know you've got a great memory, you remember.....(fill in an event he talks about all the time) so great. How about you show me how great you can remember this." If he persists with the I forget, I would say "Try, you can take a best guess at it! Even if you're wrong it's trying and that's the very best you can do. We'll do it together but you need to do the work to remember, Mommy can only be your cheerleader!" And then when he does get it, get excited, Hi-5's do a happy dance, whatever it is that will make him feel smart, and proud of himself. And that's the other thing I'd suggest, re-assure him that when he does do something right (not just his scholastic things) that it's ok to be proud of himself. Don't promote arrogance, but self esteem starts now and if as parents we foster healthy self esteem in our children when they are small, they'll be better equipt to be confident adults later in life.
Good luck! Let us know how it goes!