He may be hiding a learning disorder, vision or memory problems. He may/may not be aware of his problem(s). Your task depends on how much you are a friend but i

He may be hiding a learning disorder, vision or memory problems. He may/may not be aware of his problem(s). Your task depends on how much you are a friend but in any case rolls out to a) finding what his problem(s) are, b) telling him about them in a serious and private conversation, probably outside of work, and finally c) encouraging him to address any problems with professional help if necessary.