I've been with my partner for 6 and a half years. We also have a cat.
and neither of us has any interest in being married because for us we see it as a religious act (and neither of us are religious) and we are happy and secure in our relationship as it is. Like you, we are very open and honest about our feelings and talk through any big decisions (or small ones) in detail.
The fact that he is being so open about how he feels is really good for the relationship, although I totally understand your concerns.
I think part of it may be the old cliché of men just never really being ready to be a parent until it happens to them. (My OH has admitted that he wants kids but doesn't think he'll ever feel properly ready until it actually happens). Like you said, he fell in love with your cats and looks after them well and cares of them. Yes, it's different with a child because it's a much, much bigger commitment and changes your life forever, so you do need to be sure he wouldn't begrudge the sleepless nights, the smelly nappies etc...
What does "doing his own thing" involve exactly? I think you need to be clear on that before doing anything. If it's being able to go out with the lads every once in a while or watch TV with a beer or something, maybe there's some reassurance you can do there... Arranging in advance how you will both get "me time" for example?
Like you, I didn't take TTC for granted either, I've had a complicated gynae history and wasn't sure I could even conceive, however I did in fact fall pregnant the second month of trying, much to our surprise and shock. Sadly it ended in MC, however it goes to show that you also can't take for granted it will take a few months either, you might get lucky straight away. You need to be 100% sure now that you're both on this journey willingly.
I think you basically need to have another heart-to-heart with him and try to get to the bottom of what his worries/fears are for when you do have a baby so you can reassure him or come up with a game plan.
I think it's great that you're planning this so carefully and not rushing in to it without considering all sides.
And really sorry, the wait before TTC is so frustrating. I had to wait 2 years while feeling really broody before OH was ready.
P.S. Is there a particular reason you want them before you're 30? Is it just because that's how you've always imagined it in your head, or is it something else? Just feeling curious...