Talk to your neighbour.
If it doesn’t work, there’s seriously nothing you can do if he’s smoking inside his house.
It’s also his freedom and his choice to smoke in his house. Expecting your neighbour to totally stop smoking because you have a baby is very self-entitled.
I’m non-smoker but we all stay hdb/condo. So there’s no choice but to bear with it.
You can either close your windows and buy an air purifier or just deal with it.
Nur atiqah Sukor