Hi
He sleeps great during the night, has a first feed at 7am and then every 4 hours but he just wont sleep in his cot during the day.
At this age it would be typical to be on a 3h EASY routine, however you of course know your LO best
It may mean if you stick to 4h feeds though you need to be a bit creative with the routine until your LO can handle longer awake times.
Q1) Should I wait for two hours after his feed before putting him down even if he's yawning?
2 hours is too long for a 3 month old to be awake - a typical awake time would be around 1h20-30 including everything eyes open to eyes closed. I'm not sure from what you wrote whether your LO actually went to sleep this morning and woke after an hour, or didn't go to sleep at all? Do you have a consistent winddown routine for naps that tells him sleep is coming? Do you use blackout blinds, white noise etc?
Is there anywhere he will sleep well e.g. bouncer? I wonder if given you have another LO to look after it might be worth you trying to establish a consistent routine first with naps taken wherever he will, then training him to sleep in the crib second using shh-pat once he's used to the routine? He's likely to pick it up much quicker if he's well-rested and used to sleeping at that time - what do you think?
Also just to mention PUPD is generally only used for LOs over 4 months where shh-pat has been tried consistently for at least a couple of weeks without success
So you don't have to go down that route just yet!