I completely agree with Specialmom.
there other thing that is happening is that you are creating anxiety in your child which is why he is freezing up. The way to avoid that is as Specialmom said, make it into a game (maybe even with rewards). When we play UNO with our 3 year old grandson, it always with pretzels or goldfish. And he gets them no matter how well he does or does not do. Oh we also wear silly hats, etc.
Finally, talk with his teachers. It may be that he is doing very well at school and its just in the home environment that he has a problem due to your expectations. So talk to the experts who actually see him and can compare him to other kids in the class.
You must mean 4.9 years and not months. This is my honest opinion. Do not worry about it. Don't push, don't try to make him 'study' etc. You will make him insecure. Kids need play time, outlets for creativity, fun at that age and not so much alphabet and number drill sheets. I've got two kiddos. There not at all interested in that stuff at that age either. At all. Both went to kindergarten not reading. Now . . . we attended an honors function last night where both boys were awarded for academic achievement. Both are 4.0 students in advanced classes. Your child's self esteem is developing and if you make him feel like he isn't pleasing you or that you think he is 'dumb' in anyway . . . you will do damage. That could last a life time. Play with him. Have fun. If there is a learning issue, you will not diagnose it at home or help it/solve it by angrily asking him to do these activities with you. When he gets to school, he will begin to be assessed. Are you in the US? How does this work in your area if not? You can do stimulating things with him like reading to him, having conversations with him, taking him places, playing games with him. Try getting objects to associate with letters. Animals are a favorite of kids. A is for ape. Have a picture or plastic animal of an ape, ape, A. Then play a game after you have all the pictures of objects. A matching game. Make it FUN! Write letters or numbers on pieces of paper or paper plates, play music and when it stops they jump on the letter or number you call out. Make it FUN! If you call it studying and get frustrated? Forget about it. They won't want to participate. And remember, when my philosophy of this sounds strange to you . . . I've got two 4.0 honors students who are very different learners but who get the job done. There is a sign at our elementary school (which my kids are now out of elementary school) that says "if your kids come to school knowing all the answers . . . they haven't spent enough time in the sand box". :>))) Have fun during these years!