He is three years, three months.
Is this "verbal stimming"
My 3 year old does this too. He is rarely silent in fact. We have many reasons to believe (including professional opinions) that he is very mildly autistic and this could be one of his stims- it goes well with his echolalia which seems to be fading lately.
It can be a bit much, to put it mildly- I have issues with noise and I have super sensitive hearing.
If it it is driving you nuts, what works for us is putting headphones on him- when he is listening to music he stops talking/singing and calms down a bit. I think it relaxes him too when he is too amped up.
I do love his singing though- just not after a straight 5 hours...
If it helps, my 20 mo DD sings non stop, sung herself to sleep in the car the other day. Mostly things I sing to her ("baby mine" from Dumbo is her fave) or songs from Disney jr shows, once she picks one she'll sing it over and over and over.....so no improvising per say, just repeating things. She does do imaginative play but is reserved with strangers. I like the singing though :-)