Lots of rescue centres feed on donated food, so it can be a mixture of anything, which is why they spend hours mopping up s**t.
Darren is right a good quality dry food is best, wet food is just an expensive way of buying water, often 80% of wet food is water.
If he has been used to eating wet food, he may object to dry food at first, or may not even recognise it as food, so you need to be patient.