Didn't want to let you worry at this time of night. There are three things to do here. 1. What you can do at home. 2. Getting urgent help today 3. Following up on weight gain.
I have always had huge babies so can't comment on that, but I have had a very poorly one with a few night time trips to hospital. More times on a drip than I'd care to remember. Is he awake now? Have you tried all the tricks to get fluids in? Tiny sips, not glugs. Easier to swallow if his poor throat is sore, and less likely to throw up. Let him suck ice lollies, or just ice, or corner of a wet flannel (obviously supervise). Syringe or spoon tiny amounts off a medicine spoon. Or rather let him suck or lick it off-the feeling of the syringe can be frightening and provoke choke response. At this stage, water ideally but bribery too- favourite juice, squash. Think fix dehydration first, deal with vomit if it happens second. Above all, calmness and reassurance for him and you. If his immune system has taken a bash, he may have something as simple as a mouth ulcer. Which would account for the dribbles and refusal to really have much in his mouth. One of mine always has ulcers a week after a cold which stops him eating.
Secondly as per other posters, yes, if you cannot get fluids into him yourself, or he deteriorates, get urgent help. Don't wait for GP opening hours, ring 111. Be persistent and follow your instincts. It is frightening but amazing how resilient children are with the right interventions. I worried 'about bothering' 111 once, fearing I would be overreacting. They heard my son's breathing in the background whilst I was on the phone to them, sent an ambulance. Mid November, there's me holding my 10 month old naked child whilst one paramedic tried to get his temperature down and the other bluelighted us to A&E. Less than 24 hrs later, severe bronchial infection under control, we were back at home.
Lastly, weight gain. I think you maybe need to take a step back just for a few days. You've been through a lot over the past few weeks and clearly that's been really distressing for you. But growth/weight and development need to be looked at as a pattern, not in the immediate aftermath of an illness. Aside from the emergency appt you talked about for today, could you write down all of your concerns? Detail any dates you can remember about coughs,colds, illnesses etc. Especially any that lingered or were unusual compared to his peers. Go though his red book and fill in as much as you can. Then make an appt to see GP by yourself or with support (just without DC). Talk all of this through with them.
It sounds as though you are worried things 'are just not right', and are worried how you can help him. It may just be bloody bad luck, that he's just picked up a lot of bugs.
You and your GP sound like you are really looking out for him