I'm no developmental expert, but my baby is 5 and a half months old, started crawling just before 5 months and sitting unsupported at the same time. She also wanted to sit up the whole time from about 3 months. My LB did more or less the same at 6 months.
So I don't think its bad, if his neck is strong, let him sit. It could very well be because of the reflux that he doesn't want to lie down. Try letting him sleep with his upperbody on a adult pillow so that he is a bit more upright if this makes him more comfy.
Don't give into letting him sit in a Bumbo seat to often, off course doing it in moderation while you get a chance to cook or go to the loo is fine, but the support given by these seats is not great for development. It makes them sit with their legs straight which creates a bad posture base for sitting un-aided. The type of support it gives the back is also not good for building up the back muscles.
Do put him on his tummy a lot, with toys in front of him or on an activity mat to develop neck, shoulder and back muscles.
You can also let him sit in a box, apple boxes works well, put a flatish pillow in the corner of the box, then let him sit against the pillow with his legs bent in front of him. This way he has to support himself not to fall forward but has protection from falling over backwards. Put some toys in the box, it works nicely if you make holes in the side opposite to him and stick things through that at his shoulder level. This will encourage him to sit upright and lift his hands up instead of grabbing stuff from the floor and falling forwards.
Sounds like you have a strong little one! Enjoy him!!!