I'd be interested in why he split from his ex when the children were only 1 and 3.
A lot of fathers can't cope with the demands of small children and so don't pull their weight or look elsewhere. The trust goes, the resentment builds and the relationship breaks down. If this is what happened, why will it be any different if you have a baby with this man? Your fun, stress-free relationship will change and, rather than leaning in, he's likely to cop out.
There are two types of father ime. There are the ones who change nappies without needing to be asked, who know where their children's clothes are kept and what needs to go in the nursery bag, who do their share of night wakings without needing to be woken up, who take their children to the park and shopping alone, who don't bat an eyelid when their partner goes to the gym or goes out with friends once in a while because, for them, taking care of their children alone is something they're used to. They're not 'babysitting', they're parenting. It's natural for them to get up in the morning and make breakfast for their DC without waiting for their wife or partner to do it because that's how they roll. Similarly, tidying away toys, folding laundry and doing the dishwasher. The children are a joint responsible rather than the wife or partner's 'little hobby'.
At the other end of the spectrum, there are dads who won't change nappies because they think it's 'gross' (and therefore can't be left alone to care for their children), who can't do night wakings because they 'sleep too soundly', who can only get their child dressed if the mother leaves clothes out for them and who can only do nursery drop-off if presented with a fully-dressed compliant child and packed backpack. These dads can't take both their children out at weekends because dealing with more than one is too 'hard' and they moan if their partner goes out without them because they get 'stressed' on their own with the kids.
It is much easier for the second type of dad, who has had minimal involvement in and responsibility for their children's day-to-day life, to walk away from their kids than the first. Equal parents don't tend to bail on their kids.