I'm sorry, but I really don't understand the difference between going on date (as far as I know, "ask out" means invitation for a date?..) and just spending the time together, if you already spend time together from time to time.
If the actual purpose is to seduce him, I'd suggest to ask what does he think about sex between friends (not exactly with you, but in general, to give yourself opportunity to retreat if he thinks badly about it). ¯_(ツ)_/¯
P.S. Take all of my advices on romantic topic with a grain of salt. I am actually not good in romantic things, LOL.