And if he is already here with a work visa, I highly doubt he is 'using' you to get a family visa, as he could just stay anyway after working a number of years.
It would take ten or more years. I know this from an ex colleague.
He doesn't want to work shitty jobs for ten or more years to apply for indefinite leave to remain. Or to be able to bring his kids over, so he's not just seeing them on facetime (and maybe once or twice a year). He also probably wants to give his kids the advantages of an education and life in the UK.
More that it's unlikely in the extreme that he'd move his kids over without their Mum. That would be quite unfeasible. I think he's planning on bringing her too.
I have doubts about their relationship status. I think you could be shown oy "appropriate" communications between them, not all of them. It's weird that he's shown you, a gf if 5 months, their communications at all. What need was there; I think it was a demonstration, done on purpose to convince you/reassure you that their relationship is purely platonic but, I'd have my doubts.
Sometimes mens' wives are in on these things , I've seen several examples of it. They either have a don't ask, don't tell policy and stick their head in the sand while he "hustles" to improve their circumstances and solve their economic problems to the best if his ability,in whatever way he can ..... or are actually in on it (with an extremely stoical, pragmatic, strategic etc attitude) and the husband reassures them constantly that it's them he loves, that he's only doing it for their family, that he has to do ABC with this woman to get what they need.
This sounds a bit fantastical if you haven't seen it irl, but it happens.
He's chosen trying to have a baby with you over getting married because he's probably not actually divorced and has no plans to. He'd be concerned his marital status might actually be uncovered before m/at a wedding. The marriage could also be dissolved by you, and it might be within too short a time for it to make a difference to his visa status .... But the UK baby potentially gives 18 years of residence and strong opportunity for ILR on his own. 18 yes to get his family over too.
As a poster pointed out, why would a responsible, good father, who already doesn't have a well paid job to send money home; want to bring another mouth to feed I to the world. Which detracts from his ability to support his existing kids and is more strain on his income?
It's because he thinks you'll support it, possibly add to his support for his existing kids, and so he'll change his visa status to one that's way more secure, and may let him being his family over.
Its not normal for any man in any country to be having planned kids within 5 months with a woman he's not married to; it's visa (and money) motivated.
You're being blinded by his attractiveness/charm and your desperation to have a baby.
You're going to be the victim of a hustle if you do this.
If you want to be a knowing, compliant victim of a hustle, that's up to you.