Op has said she will not be assisting him in gaining a visa through marriage.
So he can consider it all he likes, but if op sticks to what she's said; she won't be marrying him.
Even if she did, a civil/legal marriage is different from a religious marriage. he certainly doesn't need her to do a religious marriage. He'd be keen to make sure it was a legal marriage recognised by the UK immigration service.
Her age does not stop her having a baby as the sole factor. Plenty of women have children at this age and older. It’s not ok to say she’s unlikely to have a child naturally as she’s 45. Not right at all.
I know 3 women who have had children at 45. However, while we may be able to think of women who have done so around 45; the reality is that most women are effectively infertile at 45 onwards.
The chance of conceiving within a year of TTC drops from 40-50% at age 40 ... to 1 or 2% within a year by only 43. That does not improve after 43, obviously.
Risk of miscarriage is also getting much higher through the 40s.
So, it's entirely ok to say she's unlikely to have a child (without using an egg donor) at 45 onwards... Because it's realistic.
Op also sounds like she has no intention or desire to try to have more children at this age.
I reiterate; the number of 28 yr old guys who will stay with a mid 40s woman and give up having kids of their own - permanently.. is extremely low. And that's in our culture, not even a perhaps more traditional culture like his.
People (women) like to pretend that age doesn't matter to some men; it's generally not the case. I actually think men are far more ageist on average than women. For example, women on here report their difficulty being considered by men on OLD after 50, and that they're often left with only men 10-15 yrs older willing to date them.
Most young men I know who'll date older, only do it for sex, fun, experience, home comforts etc. for a while They do not settle with older women.
The fact that there is a clear financial/immigration etc. motivation for this young man, and that his country-women say there is a culture of seeking foreign partners - with a very generous age range - to improve their circumstances; means that he may not be one of the outlier/minimal men who settle with older women and give up having their own kids (like Macron, for example).