My 5mo does too! She has done since about 2 months old. We have tried every brand and sized up and sized down too and nothing. We constantly change her bedding and sleep bags throughout the night! I spoke to HV they tested for UTI but nothing! So can see if it’s that? X
When my LG was tiny I'd change her bum before I went to bed then if she woke up in the night. X
What time is he having his last drink of the day? You could try to push that forward an hour x