Well he's 30 weeks as the calendar goes (almost) but he was 6 weeks pre-term
so we need to take 6 weeks off I think ie 24 weeks.

I think the maturing of the gut has more to do with how long they've been
on the outside, not how long on the inside. I'd suggest subtracting 3
weeks, as if he was born at 37 weeks he would have been considered full
term. Making him currently 27 weeks which is 6 months, so unless there is
a strong reason not to you could start giving him some solids.