My partner and his dad will be going home this December from US and my son's grandfather is so excited to take care of my baby. He would often say "akin na muna yang baby na yan ha" so I'm worried about his milk. We are EBFing. Should I train him with a bottle or a sippy cup for milk? He's almost 6 months.

It's so heartwarming to see how in-laws are very fond of their grandchildren and wanting to take care of these little ones. Although, in your case, it might be hard for your baby to adjust to a sipy cup/bottle if you have not even introduced this kind of feeding to your baby before. You can try, yes. Especially since you still have a few months to go before December. Just also explain to your in-laws that your baby is very used to EBFing.