How to make custard go more forward?

My pony custard is an unenthusiastic as it gets. He is very lazy, although in fairness to him, about 20x more forward than he was when I first got him! He gets a LOT of hardfeed (1 scoop of 14% high performance mix, 1 scoop of high energy apple chaff and 1 scoop of barley morning AND night!)
He also gets plenty of days off, sometimes 2 or 3 per week, I don't jump that often either.
I ride with a whip, but he HATES spurs, and as I can't use them in shows, I don't use them in case he relies on them to go forward. Sometimes I use two dressage whips, and I reward him with breaks when he's going well. The odd time we can achieve an extended trot, and 90% of the time he goes on the correct outline (also a HUGE improvement from 8 months ago)
I'm sure I haven't mentioned some things here that I've tried, but has anybody got any ideas?

Lots of variety. Take him out regulary for trail rides. Try doing some barrels with him. Play some games. Free lunge him.

Mix it up and get him a bit excited .

Work on your transitions, walk to trot, trot to canter, walk to canter, mix it up. And within the gaits, ask for a bigger(ex.) trot & then collect into a very small, slow collected trot, and then squeeze, kick, big kick & whip, till she opens up again & repeat, try at all the gaits so that she learns she needs to respect & reply to your leg.