I'd still say figure out a way to top the tank regardless of his past jumping behavior. I had a guy who was with me for almost two years disappear out of a tank that was open only for a small strip in the rear...and found his dried remains on the floor behind the tank. So yes, even the most seemingly non-jumpy bettas will jump - and they can find a way to jump through the smallest openings.
A little netting and a rubber band only costs a dollar or two and will save you a lot of heartache.
A little netting and a rubber band only costs a dollar or two and will save you a lot of heartache.