Hello
There is a so called "20 year route" which doesn't depend on the person being present in the UK legally.
So in theory, he can apply under this route and potentially get a 2.5 year leave (x4, means another 10 years for a permanent residence).
He must be able to demonstrate that he actually lived 20 years in the UK. Often it's difficult for people present illegally as they tend to avoid paper trails or switch identities... etc. But if he can actually demonstrate that he continuously (absences less than 6 months) lived in the UK for 20 years, he should be able to apply under FLR FP. Read this:
general-uk-immigration-forum/please-hel ... 78526.html
