Obviously this is coming from a person who hasn't breastfed - do your research. A child who has been nursed since birth is expected to just to go straight to a "substitute" choice... What a joke.Not all kids are the same, but there are some universal truths in development that can be pretty generally applied based on age.
My daughter is 15 months old. Solids and cow's milk have been her primary sources of nutrition since she was 12 months old, and that's fairly standard. Lots of mothers continue breastfeeding well and truly beyond 12 months, but certainly not as the child's primary source of nutrition - breastmilk alone doesn't have enough iron to fulfill a baby's nutritional needs beyond about six months, and while beneficial, breastmilk for a 16.5-month-old is a matter of personal preference, rather than critically necessary nutritional intake.
More than likely, mum is still nursing him just at night, which is pretty common, but bub won't suffer if you substitute the evening 'comfort drink' for cow's milk. It just make take some getting used to for him, that's all.
I don't think the breastfeeding will be the issue. It'll be the fact that the kid has hardly spent any time away from mum (but you did care for him for the first nine months, right? That's more than half his life at this stage). I'm not saying you'll get the overnights, but there is no harm in asking. Alternatively, you could just ask for eight hours on each second Saturday and Sunday for a period of maybe three months without the sleepover, then increase to the overnight after three months.
My daughter still nurses and she just turned 2. My ex is very supportive of this. Do you know why it's not about "me", its about the child and what makes them happy and content. Breastfeeding beyond 12 months has many many beneficial benefits for the child.