I would sit him down and ask him about it. "Grandfather, the last few times we've spoken about jobs and the future, you really pushed hard on DF and I making different choices. Why is that? Are you worried about something?"
I like this best. OP, you ask what you should respond, but there's no reason you can't initiate the convo, and there are advantages to it. It's a variation of the question-as-answer concept, and it works well.
Listen to what he says, but also have some bullet points in your mind. There are 2 biggies, I'd think. The first is that this is a different world than his. Heck, dig out those stats that say the average person changes careers (not jobs) 4 or 5 times. This is a totally different mindset than his world, where people worked 40 years at one job and got a gold watch when they left.
The second bullet has to do with both you and DF doing continuous gut checks on where you are right now, and that you are both very happy and excited about how you've positioned yourselves.
Oh...and on the child, don't get sidetracked by limited job opportunities in your area. Just ask him point blank: "Grampa, with your beliefs on family, you can't be suggesting I'd be better off with someone who didn't put parenting above all else".
Then just make sure to get closure on this convo, since you don't want repititions of it. If you have to end up telling him his questions make you uncomfortable, and/or even sad because he's worried for no reason, do so. You don't want to play whack-a-mole with this. Address it once and then make it clear you hope the topic is shelved.