What should i do?

So my son's being pretty tired all day he's 4 months and being napping way more and being super grouchy! He usually drains an 8oz bottle and today and yesterday he's only being drinking 3/4 Oz ones and he's not seeming bothered about having them I'm like force feeding him this all started after his vaccinations and I've being realising today his nappies haven't being wet half an much as they usually are ! And he wore one for an hour and it wasn't wet at all now I've just being stroking his head and realised his soft spot is indented way more than it usually is ..that's a sign of dehydration right?? I don't get how he'd be dehydrated seen as he's still having some milk every 4 hours ?? Should i contact 111 get advice?? Or leave it ? I just don't know wether I'm being overdramatic or not 😥

UPDATE : Called 111 got him and emergency out of hours doctor's appointment for 9pm ..he's currently fast asleep as his bed time is 8pm he isn't going to be happy when it have to wake him up! But hopefully doctor says he's all fine and we can go home back to bed!

UPDATE #2 so glad i took him to the doctor's he's dyhydrated !! My poor son doctor gave us medicine and I'm to keep trying to give him water and milk every hour or so hopefully he gets better or he'll have to get treated in hospital :(