My daughter is four and a half weeks now, and we sometimes have the same problem. She has been sleeping in the bassinet (flat) since we brought her home, but she tends to fuss a lot and spit up if I lay her down too soon after feeding. She definitely sleeps better in her bouncer or being held. I think it's partially due to gas issues (it's harder for them to pass gas when laying flat).

I try to alternate positions as much as I can so she doesn't get too used to one thing. We have been having her nap sometimes in her room in the crib so she gets used to it. She still sleeps in her bassinet next to our bed for now. I'd say try getting him flat for short naps during the day so he starts to get used to it. Hopefully it will get easier as he gets bigger.

I highly recommend the Fisher-Price Newborn Rock 'n Play Sleeper. It is a life saver! My daughter would not sleep on her back at all so I started letting her sleep in the Rock 'n Play Sleeper & she slept like an angel. It's safe for infants to sleep in & has them in in an inclined position. I also used it all around the house. I would put her in it & do the dishes or take a shower. I could always have her right by my side without worrying. They sell them at Walmart & amazon.com for about $40.

Does he spit up a lot? Sometimes the inclined position helps with acid reflux, he might be uncomfortable laying flat, or he may just have a hard time adjusting to a new position since he is used to being cradled or held up. I would suggest experimenting with naps during the day, see if you can put him down flat during the day and keep trying even if it doesn't work the first time. A tight swaddle, and rocking him back and forth while he is laying flat might also soothe him to sleep while he is laying flat. Good luck, eventually something will work.

Does he have gas? My doctor recommended some infant's simethicone anti-gas drops to put in his bottle. This really helped him relax and he started sleeping through the night, in his crib, right away. You could also try different formula if there is unbearable gas or spitting up a lot. Try a few things and it will get better.

Instantly upon coming home with our DD we noticed that she would not sleep in her bassinet, crib, or anything flat. We eventually let her sleep in her carrier/car seat, then bought the fisher price rock n play sleeper. At 5 weeks old she was diagnosed with acid reflux (TONS of spit up!) & placed on zantac. She is 5 months tomorrow, still on zantac. We have her crib mattress elevated b/c she still does not like to lay flat.

If my son falls asleep in our arms and I put him in his bassinet, he ends up waking back up and I have to get him back to sleep while he's in there...put the pacifier back in his mouth or rock him (the bassinet rocks or locks still). If he seems to be getting sleepy I make sure he's in his swaddle pod (he would kick his way out of the blankets I tried swaddling him, so I got a swaddle pod which is basically a really tight sleeping bag) and lay him down with a pacifier. If he's just barely asleep and it falls out (which it always falls out at some poin) he'll wake back up n I just have to stick it back in, if he's really out though he just stays asleep.
Also, when he was first born he'd start fussing a little and I'd go get him (we have a video baby monitor so I can watch what he's up to in there lol). But after a few weeks I just let him fuss a little and he ends falling back asleep after like 5 minutes tops. If the fussing turned to actual crying I'd go try putting the pacifier back in his mouth, if that didn't work I'd have to pick him up.
I don't know what you mean by he won't sleep in there..if it's just fussing let him fuss a little bit and he should go to sleep. If he starts screaming then just put him in his bassinet or crib every time he falls asleep and for each nap to try to get him used to it. Sorry this was kind of long and rambling..I hope it helped!

I don't know if this is correct or not but I have started to let LO sleep where ever he falls to sleep at. Sleep has been such an issue, so I give up! I tried to only allow my LO to sleep in pac n play or his bed BUT the struggle is not worth it!!!! So far it is working- eveyone in house seems to be getting more sleep these days.
MY LO has acid reflux and laying flat isn't really a good option

When my son has trouble going to sleep I lie him on my stomach so that he can hear my heart beat. Babies have been on the inside for nine months or so and are rather use to having the heartbeat of their mothers as well as other sounds that our bodies make. During the day do you hold her alot or a little? I notice when I hold my son more during the day he tends to sleep better flat (or on the side) at night.