I'd be getting him to practice writing silly sentences or poems with loads of alliteration for those letters, doesn't have to be anything fancy. Just something that will get him focused on that letter. Also other things would be playing puzzle type games with him using blocks or even turned over scrabble pieces and have him copy a letter that you make. Anything that is going to make him stop and say 'hang on, it goes like this' is going to help. If that's the only thing that he does, it's not likely to be a problem and he will probably grow out of it eventually. It is probably going to bother you seeing them backwards more than it bothers him kwim?