My son was quite like this until he was 5 years old. He’s almost 7 now and SO much calmer. Even a year and a half has made a hell of a difference in terms of his maturity and calmness.
My son has a stammer - especially when excited/tired - and also rambles. His stammer doesn’t affect his love of speaking, though, which is good! I find, actually, the worse his stammer/more rambly his speech, the sillier his behaviour. Quite a lot of it must come back to tiredness and overstimulation. He can also be very clumsy.
Echoing a pp in asking if it’s a stammer your son could have? Stammering isn’t necessarily the classical “p - p - p please”, it can also manifest as repeating the same word at the start of a sentence, eg my son might say, “mummy, m-mummy, when when when are we setting off to swimming?” - so once he gets going he’s fine.
Stammering is neurodivergence in itself - the brain is literally wired differently, as can be seen on brain scans, for a stammerer both hemispheres are active during speech instead of just one. So there might be handful of other symptoms from the general “pool” of symptoms that seem to be associated with other conditions, eg impulsivity or problems wit emotional self regulation.
On the plus side, it does seem to make my son more creative though, especially with language/vocabulary. Is your son good at language too, rambling aside?
Is your son bright, kind, sensitive, just needs support with self-regulation/not over-reacting?
Kids come on in leaps and bounds at this age. My advice (based only on my experience) would be, make sure he gets enough sleep, have a good routine, not too much screen time, maybe a Yoto player/audiobooks, lots of play dates to help with practising self-regulation… and he will hopefully become a lot easier in time!