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He can't open an account, I stated that already. There are other (legal) reasons besides "he has no bankroll".

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If there are legal reasons why he is barred from playing, you shouldn't be backing him. Also, by letting him play on your account, you're creating legal reasons why you should be barred from playing.

But to answer your actual question;

1. Your agreement stated a no-loss situation for you; you have no risk, because their loss becomes a loan, while their win gets cut. This is not standard, and is not backing, it is loaning. Standard backing is that you put up the $600; if he loses, you lose. If he wins, he gets 50% of the winnngs. You're loaning money -- $600 -- all of which he must pay back. You're also loaning use of accounts on which he can generate action to create the profits/bonuses.

2. In your original loan agreement, you specify that you take 10% of all winnings + bonus, with no mention of rakeback. The spirit of the agreement seems to be "you get use of my account and money, I get 10% of earnings with no risk." In that spirit, you should earn 10% of any revenue generated by their action. Rakeback is revenue generated by their action. It's revenue you would not otherwise have any of.

3. You haven't said how often you take down! This is key! Are you staking him for one session? One tournament? One week? One month? If he turns the $600 into $1200 this week, do you get your $60 now? Is it now $600 your and $540 his? If he loses back to $600, did he lose $284 of his, and $316 of your? Or did he lose his? Or did he lose yours? There's $600 left, whose is it?

4. For future reference, the negotiation of the percentage is primarily a function of how good the player is. You haven't mentioned what is pretty much the most important factor to negotiate against when backing! Of course, this is also critical to consider when staking someone, because you assume all the risk. Even in a "you must pay back losses" arrangement, loaning a loser money is a big risk, especially a loser who has gotten themselves blacklisted.