Thanks for the people who've given useful advice and helped me talk it over.
I've spoken to DS again, it turns out the pain isn't below the belly button, it's quite a way above and to the side, so it's not in the right place for appendicitis I don't think.
I applied gentle pressure, not so much as a flinch. And anyway it's not troubling him now, he's lying down and going to get some sleep.
DS is on the spectrum and does tend to play up aches and pains which he does get a fair few of. I just worried about this one as it was on the right side. He has low muscle tone (always has done) and he's prone to getting weird pains. He also finds it hard to sleep anyway, it's not unusual for him to be up at this time.
I would say it's got to be more discomfort than pain, else he'd be screaming blue murder, he's not one to keep it in!
I will monitor him, and if it gets worse, moves down or he shows any other symptoms, I'll be straight on the phone to 111. But right now, I'm reassured it's actually nothing like appendicitis. Wrong place, doesn't actually hurt that much, not unusual for him to get random pains and it's not actually troubling him any more.
I imagine some if you will say I should still call 111, but I'm his parent, I know DS, and his medical history and this is my call.
Apologies if I worried anyone.