18 month old cuddles turn to hitting/pinching/head butt

[Mymycherrypie]

My 18mo is very active. He's a big lad, more like a 2 and a half yo and he is so strong it's unbelievable. He is just an absolute terror at the moment. He runs away from me and doesn't ever look back. I tested this at the playing fields to see how far he would get and it was the length of two football pitches before he even looked back. He won't come back if I call. At baby group he won't sing to play or eat the snacks, he will just continually try to get out of the door or climb on to the stage. If he is cuddling one of us it will last perhaps 10-20 seconds and then he will randomly punch or bite. It's not done in anger and it's not aggressive. He just goes from full on loving to a full head butt in the face. He just toddles over, says hi and then bites his brothers legs. In the bedroom he will shake the bunk bed to the point I think it might fall over. He can't speak (I suspect speech delay which my eldest also had but who never exhibited any behaviour anything like this) but when he wants to, can follow a full instruction i.e. Take your plate to the kitchen or tidy up the toys. We applaud him when he's good and he looks so proud of himself. 10 minutes later and he'll have rattled the radiator off the wall.

We model good behaviour, are consistent, encourage him to be gentle, make a massive fuss of him when he doesn't do these things, tell him "no biting" "no hitting" etc he does these things and nothing makes a difference.

When a cuddle goes bad, I say no xxx and the put him down and walk away coldly. Bugger all difference and it's getting worse. My eldest was never like this. Normal tantrums and the odd hit or kick when I would put him in the pushchair or whatever but this is full scale destruction

What's going on? Is this just extra boisterous or something else.