Happened to me twice. If you can stay friends, do that. If you feel comfortable talking out your feelings with him do that, help him through it if possible. If you both are comfortable with staying best friends that’s good!
If you aren’t 100% sure you like him back you can always say you need to think about it. Sometimes you end up deciding you like him and sometimes you don’t. Never start dating your best friend if you aren’t 100% sure you love him in a romantic way