Here's the short version:
Rows and rows of standing backhandsprings, trying to go as slow and as long as reasonably possible.
Here's the long version:
You can do all of this on tumbletrak or floor or both in parallel or rod floor or whatever else may be available to you, and you may want to modify some parts as needed. Through all of this, you should be emphasizing travelling as far as possible with each backhandspring.
1) Develop what I call connecting position. Arms at or slightly below horizontal, knees slightly bent, eyes on the floor, upper-back rounded.
2) From connecting position, standing BHS, with no additional arm swing or movement of the feet. Land in that same connecting position.
3) Same as above, but -- after a complete stop in connecting position -- do a second BHS. He can stop as long or as short as he wants, BUT he cannot move his feet at all between the two handsprings, and he cannot swing his arms any lower than wherever they are when he lands the first BHS.
THIS IS THE MOST IMPORTANT STEP, and you should spend a lot of time doing it over and over until it's absolutely perfect and then keep doing it even more until it's drilled into muscle memory.
Coming to a stop between each BHS means he has no choice but to sit back behind his feet before taking off the next BHS. He can't rely on momentum from a leadup skill -- he has no choice but to (correctly) sit it back on takeoff. The point of this entire progression is to drill this detail into his brain and muscles: sit back before takeoff.
4) Do #3 a bazillion more times. Maybe add a third BHS to the sequence if you like.
5) Shorten the stop. Then shorten it some more. Then some more. Turn it into a pronounced slowdown, but no complete stop. BUT he still has to show that connecting position between each BHS. If you're not seeing a visible slowdown between skills, stop and go back to a previous step.
This may seem counterintuitive, but the goal is to NOT carry momentum from one skill into the next. That residual momentum can cover up mistakes, but without it, the athlete can much more immediately and intuitively feel errors in technique.
6) Work rows of at least two (but preferably more) BHS from a stand. Two main points of emphasis are length and hitting the connecting position.
7) Work cartwheel step-in, finishing with arms at or below horizontal, eyes on the floor, knees slightly bent, just like the position we used to stick in #1.
8) Repeat steps 3-5, but with the cartwheel step-in tacked on at the start. So it's CW step-in -> connecting position -> BHS -> connecting position -> BHS etc.
9) Repeat steps 3-5, but replace the carthweel step-in with a roundoff. Still show connecting position between roundoff and BHS. Still emphasize distance. Still slowing down and showing connecting position between skills.
10) Keep doing this and emphasizing distance, but don't bother reminding him to slow down between each skill. He'll do what athletes always do intuitively as they get more comfortable: he'll do it faster. And faster. And eventually it's a full connection, but at this point he's drilled into muscle memory that he has to sit back between each skill and stretch them out as long as possible.