I think Branwen gave some fantastic advice already. She is right, your little guy doesn't know the art of self-soothing. Waking up every 1.5 hour at night is most probably it, he doesn't know how to get back to sleep on his own. Some babies, especially sprited ones, have a hard time with this.
Does he suck his thumb at all? Did you introduce lovey, something that he can get attached to? Some moms offer their little ones a piece of clothing (or a toy that mom kept close to her skin) and leave that in the crib, so baby can still smell his mom when he wakes up.
I agree with Branwen, it may be also separation anxiety time, that would make things even worse for a non-self-soother. Increasing time spent together during day and finding another way to be close to compensate for lost intimacy of breastfeeding could also help.