What I reccommend at this age is sticking to the same bt as much as you can. And planning a 11.5 h night (or even a 11 h night at first) starting from bt. Then set the gro clock for the wu time that results. So, if you want him sleeping past 7, I would set bt at 7.30-8 and stick with it several days in a row. If he becomes ot, move forward to 7-7.30 for that day, but not on a regular basis, otherwise wu will move early (at least, for us it is what happens if I do ebt several times in a row). The other thing is being very firm that he has not to get up before gro clock. it generally works well, but you need full consitency.
How is his behavior during the day?