This is a bit late for this particular one, but on the rare occasions I do anything that isn’t my main work, if there’s money to be had, at the first discussion, I talk about how money is going to work (fixed amount of money, or hourly), then always follow it up with an email stating what we agreed.
That way everyone is clear, and if by chance someone doesn’t pay you, it is just a matter of sending an invoice and being (very) persistent.
On the other hand, it’s a small world, I bet you personally or through contacts know 90% of the people who sell these particular specialist bikes, if you’ve got tons of evidence that you helped with the design in return for a share of revenues, and you haven’t been paid, you could easily make a lot of hassle for the manufacturer if you were to publicise this, or say to notify all the dealers that you were liable for a share of profit on the bikes, or similar. It would probably be legally unenforceable, but when faced with potential hassle, chances are the small shop owners involved wouldn’t want to deal with the bikes any more, or at least would bombard the manufacturer with worried questions; if you’re selling a niche product to a small section of an industry, you rely a lot on good publicity and dealer goodwill.
I’d link him to this thread too. It’s not exactly good publicity for the bike is it.