My first question would be, is he ready for potty training? If nothing leads you to think so, then here are two sugguestions.
When I changed my babies (6 of them) it was a chance to grab a hug and kiss. twice. :) Yes, sometimes I was more rushed then others and so this did not always happen. I guess I would wonder, what is the approach of whoever is changing him? Do they feel it's just a chore and are quickly trying to get it done? With my boys, I had one song that I would sing to them every time I changed them. It was a fun song (at the time one of The Wiggles songs) and they thought that was great.
The other suggestion is, is he just trying to have control he shouldn't have? It is not always the easiest thing to change a child standing up. He may just need a consistent firm "no, your not standing, I am changing your diaper laying down" and laying him down. Boys are awesome, but sometimes can be more defiant then the girls (I have found). We definitely needed a "stronger arm" with the boys then we did the girls.
Hope this helps!