5 month old not sleeping well anymore

[IssyKuhle]

Hello I have a nearly 5 month old baby who was sleeping amazing until recently. He was sleeping for 12 hours every night without waking up from about 3 months. Taking a few naps in the day. Now all of a sudden he doesn't want to nap at all in the day. Im lucky to get him down for one nap which is usually in the morning. He goes to sleep about 7 and also now wakes up at 3 or 4am (he didn't before). then he will wake up about 6am again.
He seems to keep sucking his fingers all the time, even after feeding.

I've tried letting him cry out to nap in the day, but it doesn't work. He will fall asleep sometimes after feeding. But as soon as I put him down he is crying again. I don't know what to do.
We do a lot in the day - sometimes 2 hours of tummy time. we go out for walks (he's awake the entire time). we play with his baby gym, in the bouncer and I sing and dance with him. I don't know what more to do and why he has suddenly stopped sleeping.

He was born with Down syndrome and was 3 weeks early. but his development isn't behind at all.

please help. thanks.

It sounds very normal for this age. If you google the 4 month sleep regression it sounds like that.
Unfortunately there isn’t really a lot you can do to improve sleep massively at this stage. You can try gentle sleep training when he’s 6 months but it won’t be effective until then really.
Just keep supporting him to sleep as much as you can and it won’t be too long until you’ll hopefully see some improvement.
Maybe follow some sleep consultants on social media as they usually have some little tips for this stage.

Yes I did think maybe it was sleep regression. But thought it could also be teething because of the hand sucking? Tonight I got a lot of wind out of him and he seems a lot more settled than normal ! perhaps it was wind all along !!
I guess there could be a number of reasons.
Thanks for the response :)