To dislike it when people shorten my child’s name?

I think that's the hazard of choosing a commonly shortenable name.

I get the ick factor - I have name icks too - but apart from telling said relatives he doesn't like it, there's not much you can do except have a lower opinion of them.

As your DS doesn't like it, people shouldn't shorten it (once they are aware).

My DD (8) dislikes it when people shorten her name (think Lily to Lil) and will tell them so. If they persist and she needs me to say something, I will, but most people are happy to abide by her wishes. If family did it persistently, it would upset me because they were ignoring or not hearing her request - it's dismissive and hurtful when someone is supposed to care about you (and just plain rude otherwise).

I have a work colleague called Susan and she just ignores anyone who calls her Sue, to the point I get second hand embarrassment.

Maybe your son could do that.

I think you can. Both my DH and DD1 have names with very obvious shortenings, but neither get shortened by anyone who knows them, as they know they don't like it and respect that.
We have much more issue with actually getting people to use DD2's preferred shortening rather than her full name.

I’m afraid that I consider people incapable of dealing with a full name just a bit thick, especially when they know perfectly well that you don’t shorten it.

Thick and not a little rude. It’s not affectionate at all, especially if they get the arsehole when corrected.

If its a name that traditionally gets shortened then I kind of get that, I am the worst for shortening peoples names, think Christopher to Chris. However only if it makes sense, my DS is Henry and he got called Hen the other day 😅 definitely not. Whereas my other DS is Edward and he gets everything from Ted, Teddy, Eddie, Tedstar, Edwardo, Tedward. I could go on.
I think it's just a sign of familiarity and affection. Unless you really don't like the NN, I wouldn't let it bother me. How old is your DS? Once he's in the big school his friends will probably NN him anyway.

How old is he?

Honestly I would try to let it go. If it were a weird nickname yes you could nip it in the bud, but you are never going to get people to not shorten a name like Thomas to Tom. It is affectionate and if it really is a name like Tom or Dan probably done without thinking.

Ultimately if your son doesn't want it to be shortened, then it shouldn't be.

My daughter has a quite traditional girls name, but shortened it to the traditional boy shortening of a similar boys name rather than the more usual girl shortening. Her pre school teachers and grandparents absolutely refuse to use the shortening, saying 'it's a boys name'.
Luckily her school teachers are very happy to respect her wishes, and she is now on the school systems in her shortened name. She is very individual and this name absolutely suits her.

I assume that you’re in the UK where I think it’s seen as a sign of affection. One of my kids has a one syllable name and he was given nicknames too so you can’t win.
I understand the frustration - especially if ds says that he’s Thomas and not Tom so I’m not sure of the answer.