That is heartbreaking. If he is friendly could you pop him in a cat carrier and take him to the vet to see if he is microchipped?
If he isn't leaving the garden even though you're not feeding him, then I don't think you can make the situation worse by providing food - it is horrible for you, but it is a temporary situation, and watching a cat try and survive by eating slugs is just - surely - unbearable?
There is tons of advice here for dealing with 'found' cats, though since he isn't going anywhere else I wouldn't personally bother with the paper collar bit:
http://www.catchat.org/lost.html#foundIf you look to see which rescues cover your area, I'd be haranguing
all of them, getting him on waiting lists for a rescue place and getting hold of a neutering voucher asap if he requires one! It is likely that he won't be top priority because of all the kittens and pregnant strays around, so it's a case of persistance and patience. As well as food and fresh water, could you provide him with a bit of shelter in the garden?
http://www.catchat.org/adoption/index.htmlIt might be worth trying Zylkene for your upset spraying cat, in addition to the Feliway.