Controlled cry

I have started the controlled cry with my 15 month old as he’s never been a great sleeper I tried this when he was around 8 months old but it didn’t work with him then. He is now 15 months old and has never slept through the night once and he seemed to get worse with sleep and waking constantly through the night the older he got. We’re now  on about day 4 of the controlled cry and he will now settle of to sleep by himself with very little fuss which he has never done. The first night he cried on and off for about 40 min before he fell asleep but he slept for a good 4 hrs which he has also not done before the second night he did the same cried on and off for about 40 min but then slept for 5 and half hours before he cried out. The 3rd and 4th night he has gone down with none to very minimal crying and will settle himself to sleep but is now constantly waking through the night every 1hr to 2 hrs and sometimes every half hour to an 1hr where he will initially cry out and we have to lie him back down and tell him that it’s night time and it’s sleep time he will then settle back off himself again as soon as we leave the room with very little to no crying so he’s not really having a strop or tantrum to be with us or to be settled off by us as he will fall back to sleep with very little fuss but still constantly wakes. Does anyone know if this is normal or should he be sleeping for much longer periods with the controlled cry by now?