I wouldn't even consider it initially. I visited a gf recently with my then 7 month old. Her 3 year old was incredibly good with my DD, but she was eager to show her all of her toys so my DD ended up being swamped in toys. I could just imagine a 3.5 year old doing the same thing through the sides of the cot, ending up with your new baby being surrounded with potential dangers. And even worse would be any issues that might come up out of jealousy - its your DSS's room and he might be upset at the thought that this new baby comes along and not only takes time away from him and his daddy (and you) but takes his room as well. I would be keeping the new baby in with you for as long as possible and then just assess the situation down the track as your watch the children interact.