My 17 month old son keeps dropping himself to the floor?

Sounds like he is messing you about and a lot of children do it. Try a children's harness when you go out and if he drops lift him up using it and he will soon learn not to do it as it will get him nowhere. Also, when he is walking just holding your hand and he does it then warn him by saying 'do not drop to the floor or you will put on the harness' and then if he does it again you say 'no' and put him on the harness and let him scream if he wants and just ignore it. He is most likely just at the stage where he is pushing the limits to find where he stands on your terms.