Steve, use Scowan's idea (fantastic by the way, thanks scow!)
He should be leashed and tied to you at all times when you are home. Our concern right now is not whether he gets enough play time but getting him housebroken ASAP.
Any chance of being able to come home at lunch for the next 2 weeks? Or maybe a pet walking service, someone you trust to let him pee somewhere around the middle of your day?? It is asking a lot of him because he is a super large breed and therefore develops more slowly. By 20 weeks he is certainly capable of holding it for 10hours during the day....
Also make sure there is NO bedding in the crate to absorb any pee. If you bought the super huge Vari kennel, cut it in half and see how he does.
let me know how its going.