If your daughter was 6-9 years old too, I would say "he's just a kid" too but have a talk w/ the mom and just ask that she remind him that your daughter is too young to be "kissing" him/explain boundaries.
Since he is 8 and your daughter is 3, although it could just be him being a kid, I suggest you have a chat w/ his parents. It may be nothing but at this age, five years is a lot and it could be something. Hopefully it is just him being mature but young kids are hurt all the time by older kids and you have to protect your daughter.
I would say, at this point, it is ok for your son to be friends with him but not to have him around your daughter unless you are right there.