Ask once, then tell, then MAKE HIM!!
"please pick up your toys"
"No. You pick up the toys"
"Pick up your toys now"
"No. You pick up the tops"
Go to him, take his hand and with his hand in yours, pick up the toys, without further communication.
Go calmly, not storming, not angry, just matter of fact. His hand WILL be used to pick up the toys.
When toys are picked up, say thank you.
I had to do this stuff SOOOOOO much with my 2. And still do at times - and they are 7 & 8 (don't panic, they're autistic, normally kids HAVE grown out of it by then ) and we still have to physically take them and do the hand over hand stuff. (dh more than me cos they are getting too bloody strong now, I can't shift 'em!! Bugger knows what we'll do when they can overpower dh!)
Once you have given an instruction, it MUST be carried out. It's not a fight between you, so don't let him turn it into one. You are mum and you have given an instruction and you will be obeyed.