Have you tried massage?
Maybe after shower or bath, turn the lights down (we have just a lamp on) dry off then a massage, it doesn't have to be long or involved. He might like just lying on his tummy while you run your fingers down his back.

Arlea likes having her feet rubbed And the 'rule' is, even if she's not quite ready for bed after dinner & shower, we stay in the bedroom with her, lamp only on, and have quiet time until she's ready.
Can you enlist your DH to keep an eye on DS1?

Although she is a bit younger than your little man, jsut a few ideas...also you could try calming music? we did this with DD1, worked well.

HTH