I think you should first find out the rootcause of the problem. Why is he being forgetful.. is it b'cos of his lack of interest, the concept is not clear, is he finding the syllabus difficult.
while learning new things, does he show interest in learning or do you have to force him to complete his homework/ or learn? If he doesn't show interest, find out the reason. try to change the method of teaching/ learning. even the time at which he sits to do his homework or study also matters.
Does he understand the concept clearly? You can check his understanding by reviewing the lessons asking questions or asking him to solve the math problems. also find out if he needs your involvement all the time while doing homework? try to make him learn indepedently as much as possible. if he makes mistakes, tell him that are mistakes and let him try to figure out which are wrong. try to restrain yourself from correcting them yourself.
if he is finding the syllabus difficult, see how you can help cope up with it. he might need some extra help
try to find out his learning style. If he is a visual learner use lots of books with illustrations or show videos that are related to that topic. try to use lots of pictograms. if he is auditory learner, make him listen to someone read the book for him, keep discussing about the topic. if he is a physical learner, try to demonstrate the concepts.
Also revisit the old concepts whenever possible. if he has learnt about Animal life cycle, talk about it sometime later. you can try conducting a quiz though not a formal one. play a game when you 2 can ask eachother questions to find who knows more.
talk to his class teacher and try to find how she can help in the classroom. If you are still worried, then get some professional help.