Just consider that maybe if your horse has forgotten the cue for 'go' he may also have forgotten the cue for 'whoa'
If you do decide to try the whip approach then be sure to be in a safe enclosed area - just in case
If he has no clue what a leg aid is then no amount of spurs, crops or kicking will get him to do what you want. Same as asking a person "Where is the bathroom?" if they only speak Cantonese - you'll never get the response you need.
I'm not sure if you or your daughter will have the experience to pretty much start his training. This is what I'd do (and I'm sure lots of more experienced people on here would do it very differently). I'd be starting to teach pressure at the girth on the ground (getting him to move off pressure on one side), as well as getting a solid set of verbal cues happening on the lunge. At this point I'd also be starting his lateral mouth on the ground, as I'm betting he may not have that either.
Then I'd start ridden work again and accompany the verbal cue with leg aids. Simple transitions. When not giving an aid I'd have NO leg on at all, to prevent the horse getting confused at this point. If required, reinforce with a crop, but really give him a chance to respond to the first cue before upping the pressure. Ask, tell, demand. Gradually I would be able to phase out the verbal cue. The most important thing at this stage will be to get him going forward in a free manner.
Downward transitions can be worked on by requiring an increasingly fewer number of strides to go from walk to halt (or trot to walk), and using the voice cue to reinforce once again. Just requires repetition, but I'd keep it interesting so he doesn't tune out. But if he doesn't yet have a solid halt on him I'd be wanting to work in an enclosed space because if he starts going forward he may not want to stop. That's where having given him a lateral mouth on the ground (flexing) and testing it under saddle comes in very handy!
But anyway, that's just how I'd approach it - especially as he's lungeing nicely.