I have an almost-
I would just say "X (your son's name) would like to have Y (the other 6yo) over to play. I don't see the need to invite them both. I have an 8yo and a 6yo. The boy next door is also 6 and I wouldn't expect his mother to invite my 8yo over, although when he comes to my house he plays with both of them (or tries to). I think at this age (and at the 4 and 6yo age) two years is a HUGE gap in age/maturity.
My cousin use to do this to me all the time. One of my nephews was a year older than my oldest DH and the other one was three yrs older. She would never let the younger one come without the older one. When they were younger (they are all teens now) the three-year age gap was too much. But she insisted that the boys could not be separated. I think she just wanted to get rid of them both at the same time.