I deal with this a lot because I have a 4 year old who can't keep up with his 5 year old brother. I usually acknowledge that yes, he is slower but he's doing his best and he is awesome for his age. You can also compare him to what he couldn't do before. We did 2 lists with Yasin and Imran. When I was a baby I couldn't.... Now I am 4/5 I can.... They both liked seeing a list of all the stuff they have achieved (walking, eating real food, talking lol)
I think it will be less of a problem as he gets older because soon there will be smaller kids in his room and it will be his turn to be better but maybe you could encourage him to be more gracious about it than the big kids.