A serious suggestion - change the reward system so he is far less likely to self-sabotage. Instead of "days in a row" needed to earn the reward, simply make it a reward which is earned piece by piece, day after day, so after x number of days, total, he earns the reward. That way, you could have the reward a 7 day reward by maybe he earns it by Wednesday next week, for example.
Otherwise it can seem too unattainable and they can stop trying.
We're using a reward system to encourage difficult child 3 to get enough schoolwork done. He has 8 school subjects, with worksheets for each subject needing to be done each week. I calculate that if he does two worksheets per day, each of five days, that will put him ahead by two subjects. In reality it is very hard for him to get two done per day, especially later in the school year. (some of these "worksheets" are an inch thick).
His reward is a large packet of Maltesers, for each day he gets two worksheets done. He gets double reward for each subsequent worksheet he does on the same day. That can add up to a lot of Maltesers, when the work is going easy, so sometimes I will use "partial credits". Last year we bought some coloured cut-glass crystals for about $5 each. He could exchange two credits for one crystal. He was happy with this (we worked out the rules between us, that way he 'owns' the reward system too = greater compliance likelihood).
The chances of difficult child 3 completing 10 worksheets in a week - it just hasn't happened. Circumstances plus his own lack of stamina see to that. But if he can average 8, he at least keeps up.
We've also modified the rules (for a kid who refuses to do homework) - if he completes worksheets at night or on the weekend, he can credit them to the adjacent schoolday of his choice. Alternatively, a worksheet done on a weekend earns double credits.
Sometimes he only completes part of the work. But if he completes part of the work on a Friday, and finishes it over the weekend, he is credited with having completed it on the Friday. And a double bonus - he starts fresh work the next schoolday which increases his chances of success.
It sounds complicated, but this has evolved through involving him in this process, and it is working.
He's currently negotiating to be allowed Wii points cards as rewards. I need to talk to husband about that, to see how feasible that is for us.
I hope you can glean some ideas from this.
Marg