What about a Warmie? They are gently weighted stuffed animals you heat up and then lay on your baby so they feel like they are being held/touched. Maybe add something that smells like you as Naz suggested too.
Agree with Naz. Your LO is used to your smell, the heat, and the weight/pressure of being against you. You'll need to replicate that environment as close as possible. I wouldn't immediately grab him when he starts to fuss (just comfort him, rub his head, kiss his cheeks, let him feel your touch). If he cries yes I think you should absolutely pick him up to comfort him but try that process over and over again consistently if you can. My daughter sleeps the absolute best when she's snuggled up next to me but has no problem sleeping through the night in her crib since we started using a sleep sack when she was 3 months. There are so many out there (we use Nested Bean) but as soon as I started using it she slept perfect every night (she's now 6 months). Good luck to you! - Mom of 5 🥰
Have tried maybe making the sheets smell like you? Or wear his night clothes under yours for the day and put him into bed. I would start off by making him sleep in his cot during day naps, and be consistent and patient, he’ll eventually learn that it’s his sleep environment. Another tip would be, white noise or stroking his head for reassurance. I used the pick up and put down method on my little one but it takes a lot of patience x