S.F.
I have an 8 month old as well and luckily he has learned our routine very well. I do feel for you, as my first son had colic. We drove and we walked the floors and I thought I was going to loose my mind!
I was just wondering, when you go in to check every 10/15mins, do you speak to him?
I share a similar routine with my 8 month old. I do my best to have him in the bath by 7pm, he has a bottle while I read him a story (this is since he can hold his own bottle now), we may snuggle for a bit after the book and bottle and then bed by 7:30 or 8pm latest. If he cries I return to the room in 10 minutes, I will straighten him out (that is if he's in the crawling/standing position) and then tell him “it’s bedtime”, give him another kiss and leave for 10 more minutes. If he continues to cry, I would return every ten minutes to put him back to sleeping position and I DON'T SAY A WORD to him. It seemed to be the key for him and now he knows the routine very nicely, tho it wasn't easy to to not speak to calm him. At first sometimes I would have to pick him up to calm him down because he would be crying so hard but I still would make it a point not to say a word. (the first night i went in at least 4 times, second night 3 and by the 3rd night i went in only once. now i usually don't have to go back in at all. once in a while i will but it's never more than once)
I don't know if this will be any help but it might be worth a shot since you are doing basically the same thing anyway.
Again, I do really feel for you; I wish I had some better advice. Good luck.
Sincerely,
S.