I would have said with his socks on or with his socks, not in his socks. I guess it's maybe a way of speaking in your area, or perhaps it's a way of speaking I've not really heard of and neither the incubators. So I'd recommend you report this translation and maybe it will be deemed correct.
I tried two answers to this, I think both are okay in English. 1) With emphasis on 'He' the question could be phrased "He sleeps in his socks?" 2) "Does he sleep in his socks?". As said in another comment 'with' implies cuddling rather than wearing so 'in' is better in English. I clicked "Report" for both.
It doesn't sound natural in English to say sleep with if you mean he is wearing his socks in bed. You would sleep with your partner, your dog, your teddy bear - implying you are in the same bed, or the same room. With an item of clothing you would sleep in it. At least where I come from.