My boy got that when he was about the same age. The corner of one of his toes sort of went like a little blister that was yellowy...like it had a bit of puss under it. His big toe ended up going all red

I would pick at it and squeeze it to see if it was sore and if I could do anything about it myself. I asked the pharmasist when I was in there if there was anything they could recommend and he told me off for cutting his toe nail too short! I hadn't even cut the at that stage...they just grew like that! Boy was I annoyed with him.
Anyhoo....if it isn't bothering him, then just sort of massage the skin in warm soapy water (actually Im not sure it even has to be soapy), and it should come right...
If it is bothering him and making him upset, then take him to the docs.