Hello,
I would think that with a problem like this, ie no probs getting to sleep, but wakes up after 35-40 mins, your best bet is wake-to-sleep, rather than pu/pd. Waking, and then going back to sleep after a lot of crying is a bit of a disturbed nap, and with wake-to-sleep, you try and avoid the waking in the first place.
If you don't know what it is, there's a great section in our FAQs section (at the top of the Naps forum). If you have any problems with that, let us know.
If you really do think he's in pain, then you're right not to use pu/pd for the time being - it's a sleep training tool which should only be used when all the circumstances are right to do so.
he woke at 10 again screaming as if in pain, he still would n't take the paracetamol. This time he took some water. I then held him until he calmed and began to get sleepy. I put him into the cot sleepy and then stayed with my hands on his knees till he was asleep.
This sounds great to me, especially if you suspect he may be in pain. He's still falling asleep in the cot, not in your arms or on your breast, and that's what you're aiming for.