Quick answer - he does get equity at the next funding round. What he gets is the amount of equity that $150,000 would buy at that valuation.
So if someone else invests $250,000 for 25%, that's a valuation of $1m. Yuri would get 15% at that valuation. He'd get 7.5% at a $2m valuation and so forth.
What happens if there is no exit or "liquidity event", but the startup is profitable and never takes additional funding? When does the $150K become equity?
Some notes have automatic conversion at a given valuation (usually the 'floor', which in this case does not exist.) Others have no such clause, presumably under the incorrect assumption that a "liquidity event" will always happen if there is a success. I've dealt with both types of notes before and always insisted on adding an automatic conversion clause.
Might there also be some far-out repayment date — say 10 years — and a small interest rate (even without a discount)? Then, if the company becomes a perpetually profitable independent growing concern the loan is still eventually repaid, though not with the kind of equity conversion and appreciation the investor is really hoping for.
"Startup fails" is the reason why these companies have the so-called "limited liability". They file for bankruptcy, founders get on with their lifes, investors see nothing. This is, in fact the vast majority of the cases, and the reason investing is called "high-risk, high reward".
So if someone else invests $250,000 for 25%, that's a valuation of $1m. Yuri would get 15% at that valuation. He'd get 7.5% at a $2m valuation and so forth.