helenlouisey, if the rice you are planning to use is intended to be in addition to his breastmilk, then mixing with water is just fine - you are adding calories to his existing diet, without taking away any nutrition.
If you wanted to use milk instead, then cows milk would be fine too - guidance is that it's fine from 6 mths but not as a 'main drink' and I think common sense would urge you not to worry about your baby being 5 mths and not 6 mths. It's not that cows milk is harmful before that age, and the amount you would be using to mix baby rice is pretty small.
Full fat cows milk has less vits in than formula, because the vits are added in to the formula - maybe your dh is a bit iffy about formula because it is a processed, manufactured product? In a baby who is still predominantly breastfed, I can't see it makes much difference either way, nutritionally - do what seems most convenient to you, and that's probably water, or expressed breastmilk, in the end.
However, my understanding is that the reason many authorities suggest weaning a little earlier for prem is because of iron - they didnt get the iron stores boost that comes towards the end of pg when the baby is in utero. So if this is your concern, then you could ask your HV about the relative iron content of your proposed options and, of course, whether it matters or not.