M.G.
I would say, that if you have no other issues with this daycare, then do not switch him to another one. It does not sound negligent on their part because a kid throwing a toy is not something you can just prevent every time. Did his injury even require medical treatment? One of your rights is to have it documented, kept in his file at day care and a copy for yourself in case it does become a pattern.
Your child may not be able to verbalize what happens to him there, but at 20 months, you should be able to pick up on whether or not he hates it or is scared. Since this is your only child, count yourself lucky this is the only 'major injury' you have had so far because there are SOOO very many more to come. Some while he is in your care, some while he is in the care of others. Kids get hurt and that is just the way it is. Luckily it wasn't more serious, but you cannot just uproot your child every time he gets a boo boo. Seriously. Let him stay there and have some consistency and understand, he is gonna get hurt again. And again. and again....You certainly do not want to over react and become that 'mother' every one at the daycare dreads. You know what I mean? Trust me, by your third child, you'll look back on this and blow it off alot easier than you can now.