I never saw the value or advantage in 'rewarding' children for behavior that's expected of them. At age 5, he should have had his bathroom skills down pat (and for much more than 6 months or so). This child isn't being challenged at all to live up to reasonable expectations of him - as long as you treat a child like a baby, you're going to have nothing but a baby on your hands. This is all assuming this child does not have an organic/medical reason why he'd not be able to function like a normal 5 year old in this regard....
Just impose reasonable rules - like 5 year olds don't 'forget' or don't 'not make it' to the bathroom. 5 year olds are MORE than capable of, smart enough to consistently make it to the bathroom and to have the necessary skills to manage their own bathroom functions. Period.
And, once the rules are established, then establish consequences that will incur if the rules are
- denial of privileges, isolation, some other meaningful and relatively painful result. Then, when he inevitably breaks the rule to test your resolve and assert his control, you totall surprise him by following through with the consequences and sticking to your guns until you get a result you want. And the next time he breaks the rule, you totally shock him by again being consistent - actually, merely by being the parent, not the buddy, not the friend, no the 'peer', but the parent, whose ultimate responsibility is to raise a child who can function up to his potential and beyond.