Worried About My Brother's Fasting Without Praying

My brother has been keeping his fasts but he doesn't pray at all. It's not like he's busy with work or anything - he's mostly free at home, but still doesn't make time for salah. I honestly don't understand this. I keep reminding him gently, but he just ignores me or changes the subject. I don't know what else I can do or how to convince him to at least start praying. What's the benefit of fasting if you're not establishing your prayers? Doesn't salah come first in our faith? I keep wondering if his fasts are even accepted without prayer, and that thought makes me really sad and worried for him. I just want him to understand how important this is, and it makes me feel so helpless.