I think you're doing great amberj!

Both my girls went through stages of doing the same and it is just a phase that, if consistent with your response, will get a longer nap time out of him eventually. You seem happy with the self settling idea so maybe just extend how long you leave him to self settle (monitoring him of course for harder cries etc) but if its just a general whinge, he could actually be in the transition between lighter sleep back to deep sleep so you probably don't want to stimulate him too much with fussing iykwim? Just a thought then, it could take him a good ten minutes to settle himself so if after this time he is still fussing, you could do all the necessary checks, eg nappy, hot, cold etc and keep your visit quiet and quick and then pop out to let him attempt to settle again.

Sometimes it works, sometimes it doesn't. I do try to keep him in bed for an hour, even if the last 20 minutes are me going in and out sticking the dummy in. I have a vague idea that maybe this will give him the notion that he needs to stay in bed longer....but then, its probably completely arbitrary to him and he just knows if he whinges long enough he eventually gets up!
I think you're right! So another thought is maybe don't worry about the dummy as it sounds like this could be keeping him awake after all.

I think its important that you stick to your method of, "mummy decides when nap time begins and ends" as this with minimum fussing will also teach your little man how nap times work This worked great for my girls too! hth