What is the current going rate in your area? Are you at, above or below this? If you above this then business is business and he's bound to ask for a reduction to current market rates. If you're happy you're charging the going rate or below, then he won't get a cheaper apartment by moving so you have to then ask yourself, is he genuinely having difficulty meeting the repayments or just 'chancing his arm'? If you believe he genuinely cannot pay the going rate, ask yourself are you prepared to take below the going rate to avoid searching for a new tenant if you believe this will be difficult (be honest with yourself-how many aimilar units are to let in your area?)/because he keeps the place particularly well and is a trouble free tenant (worth a lot)/because you feel sorry for him (bad business of course!)/etc. etc.
There is no right ir wrong answer. You must decide what you are prepared to accept. I doubt he'll be entitled to rent supplement but the SW guys on here might know for sure. It's a HSE payment not a SW one AFAIK so maybe.