Hi Sens,
I would move him in with his brother and do controlled crying.
Looking afer lac is not the same as your own and you need to fuction, if you are not happy then you may give up fostering after this or not take on any more young ones which would be a real shame.
You need to look at the bigger and longer picture and set up stratagies that will work with your family long term, with this placement and the next and the next.
I have always breastfed, co-slept and done attachment parenting with my own so it was a big shift to look after other peoples kids in a differnt way, but it does get easier and the lac thrive with routines (which i hated with my own kids)
That is not to say you don't care about them or neglect thier needs, it's just done in a different way.
I am very sleep deprived at the moment as have a 10 week old who is not settling into a routine and is up quite a bit in the night, but she is only a tiny newborn and I would never advocate cc for that age, but you lo is over one and at that age should be capable of sleeping through the night.
You may need to check there is no medical reason for his waking or nothing traumatic in his history to explian it. But if its ordinary toddler waking then i would recommend being firm with him.
HUGS to you. 