I would follow the baby, not the clock. Don't keep him up and risk becoming overtired just to stay on clock time. If he is tired, let him sleep.
I think, at 7 months, if he is still doing the 2 naps a day, you should try and have him asleep by the 2 hour mark, which means starting your wind-down at the 1.5 mark or 1.75 mark. Aim to have him sleeping by the two hour mark