Young cat crying and yowling - illness or stress?

[LittleCrow]

Excuse the detail but I'm not sure what's relevant.

I was away for 72 hours whilst family were at ours looking after the DCat and DD as I've just had a baby. DH returned without me and our newborn less than 48 hours after but when I came home yesterday, relative says DCat is not eating wet food, only dry food and free access to water. DH says it's been that way for a few weeks despite trying all kinds of wet food but DCat has been eating enough dry food. When I take a look at DCat, he seems visibly thinner which I've not really been present enough to notice over the last few weeks. That being said, my late DCat would get quite slim in the summer as he'd stay out all day.

Last night DCat looked at newborn and paid little notice other than to watch cautiously and then walked away. Tonight newborn is upstairs with me not yet making any sound, DCat comes up and lays just outside the door crying, he comes in the bedroom and for the very first time, starts yowling at full volume like I've never heard him do before (kind of like a mating call but he's been neutered). DCat doesn't acknowledge newborn but gets up and cries on the end of the bed facing away from us.

DH calls him and gives him some tuna to get him eating something other than dry food, he eats a few nibbles but not much. DH then goes to get him some of his favourite food from the shop to try that (although he keeps going off of it intermittently). Whilst he's out, DCat yowls at the front door for several minutes, newborn starts crying so DCat runs up again and takes a look at newborn, yowls again then joins the chorus with newborn crying.

DCat still doesn't eat his food and just licks at the top half heartedly.

Could this be due to stress/change within the household or more likely to be a health issue?

Longtime cat momma here. There’s a whole list of things that could be wrong-stress being one of them yes.
However, thyroid or kidney issues can also be the culprit of excessive vocalisation and lack of appetite.
A vets visit is entirely necessary

Anything abnormal should involve a vet so please make an appointment.

Also being neutured doesn't always take away the mating instinct, it just stops the making of kittens. I only learnt that recently.

My dcat sat on the stairs and howled the first night we brought dc's back.
He adjusted over time.

He needs to see a vet as soon as possible for a health check. Weight loss and sudden excessive vocalisation are both signs that things are not quite right.

Once he's been "cleared" by the vet then yes, you need to look at the possibility of stress. Some cats are okay if you try things like Feliway but others can't cope with the noise and unpredictability of children and will struggle long-term, sadly.

DCat started vomitting, the vet thinks it's stress as weight is fine for his age. They're not concerned he's gone off of wet food in current temperature as he has water and dry food.

Tbf he's been on high alert with the 5 year old and baby, choosing to be near them even when they're upset despite having places to hide. He's obviously not coping as well as we thought though.

Any suggestions for stress apart from Feliway?

For context, he was 2 months when we got him, he'd been with a family who had a baby and children was perfectly fine with our DC who was nearly 4. He's always had a few hiding places and a room to hide away in if need be. He's used to DC being loud at times and takes himself off out or upstairs.

I'm hoping he'll adjust eventually due to his history with children. It's horrible seeing him look so unwell but if it came to it, we'd have to consider rehoming if he can't cope with it. We'd just like to avoid that as he's a family member.

Lots of attention and play when you can dedicate a bit of time to him (difficult with a baby and a child, I know, but encourage your 5 year old to join in some gentle play too)
As long as they feel reassured, cats are usually very good at adjusting to new family members.