I think they will work without a SIM but you need one in when you activate them after a factory reset. Why not just get a PAYG sim for it?
Both my kids have 5s, good phones and they are oldest Iphone that will work on the latest IOS. We put a glass screen cover and case on ours and they sime pretty tough.
This may sound silly but if it's just for him to play on then I think you would be better off selling it and buying him a tablet. In good condition with all the accessories they can make £100+ on ebay especially in the lead up to Xmas