I recently bought him a large heavy wooden toy chest, the lid is relatively heavy - it was heavy enough that I didn't think he'd be able to lift it, but he is freakishly strong for his age. He's closed it on his hand a couple of times before and whinged as he couldn't get away, but he's never hurt himself until today, when he dropped the lid from some height onto just his thumb. (He's 16 months old by the way). He cried for an hour, and now has a black bruise on his thumb, so obviously this can't continue. There is nowhere else the toychest can go other than his play room, absolutely no room elsewhere. I've tried putting a heavy weight on top of it but he merely slides it off, and obviously can't do this again in case he slides it onto his foot. I honestly can't see a standard cupboard lock working, if he can lift that heavy lid I'm quite sure he can tear one of them open. So I'm at a loss, any ideas?

Get hinges from your local hardware store. They're easy to install and if you ask for help and tell them what you need it for they should be able to help you. Also if the toy box doesn't already have holes see if someone can put some in even if it's in the back where they aren't seen. If he tries to reach for something inside and that heavy lid comes down on his head it can knock him out and into the chest. Then with no air holes he can suffocate.