I'm really worried about my dog, he's a two year old king charles cavalier. Since last night he's been yelping when he moves or if my other dogs go near him. I can't think what it could be. We've got a halogen heater and he caught his tail in it yesterday morning, he didn't yelp or anything he just licked it a bit. At first I thought it might have been some kind of delayed shock or something and he just needed a cuddle. But he's fine with me touching his tail, so I don't think it's that. He's fine when he's laid down and when I stroke him. The only other thing I can think is that he could have swallowed something but if he has I don't know what. He seems to be in pain around his chest or his stomach, maybe his legs but I don't think he'd let me touch his legs if that's what was hurting. He's been laid on my knee and he didn't yelp but then he stretched a bit and he yelped again. I haven't tried him on any food because I don't really want to wake him up or make him move if he's in pain and I'm not sure if that's what you're supposed to do if they swallow something. I can't bear to see him in pain but my dad seems to think he'll be fine if its just something he's swallowed because he says it will just pass through. What else could it be?

I'm sorry but your dad is wrong.
Yes it may pass through but I wouldn't take the risk if he is exhibiting those symptoms, I'd already be at the vets.
If he swallow something sharp it may have pierced his intestines, stomach or bowel.
If it is not sharp then it may have blocked off something and would be extremely uncomfortable, not to mention deadly if left untreated.

There is also the chance it is something completely different but only your vet could tell you that.

My normal response would be to feed cotton wool and bread to cushion the sharp object but seeing as how he has been like this for quite some time I don't think it would help.

Convince your dad to get him to the vet as soon as possible.