He sounds completely irrational.
He's decided he wants his own kids.
There's next to no chance that would happen naturally.
IVF would be hard (on you), expensive, and no decent clinic would advise you to use your own eggs. So you'd be a vessel for his & an anonymous Spanish student's biological baby.
His sperm is still a risk.
And all this when you have 3 kids under 10 to look after.
He's not thinking of your kids best interests, is he?
He's not thinking of yours either.
You've only been involved with him for a year. How long ago did you split from your children's father, and did they have to adjust to being kids of separated/divorced parents, and move home (?)
Why, after a year, are his ridiculous and unrealistic ideas, a potential factor in their lives?
This would be too soon even if you were both of a suitable age.
This man lives in cloud cuckoo land.
Wants his own kids but when you say (correctly) the relationship will have to end, he doesn't want to.
He's not good with reality, is he?
At half a century old.