daytime sleep

[jandrsmum]

Hello
I would love it if anyone could help...DD who's three was an angel baby and sort of got herself into a routine. DS, 12 weeks, is lovely, but not as well behaved! He just is very unhappy sleeping in the day. I can get him back to sleep at around 9am, but he wakes up about 40 mins later in a fury and screams until he falls asleep again (via a cuddle or a walk, usually), so I'm guessing he needs more sleep at this time. Do you have any suggestions?
But worse is the fact that, although he's obviously tired, he won't sleep at lunchtime (or any time later in the day) and will just cry in his cot when I put there (even though he's yawned etc). He will sleep if I cuddle him, but I can't do that all the time because I need to spend time with his sister. Did anyone else have this problem as I could really do with some help? I have tried leaving him to cry, but he just goes on and on and doesn't cry himself to sleep. I thought he might be hungry, but started to feed him and he then fell asleep - of course....
He goes to bed happily at night in his cot, by the way.