I am afraid in such a situation, of a temporary residence permit, your future spouse will have to go through the usual UK route. This means:
Step 1 - obtaining a 'family of a settled person' visa, which can be continuously extended; and
Step 2 - after 5 or 10 years on this visa, apply for permanent settlement in the UK.
The first visa is described here: https://www.gov.uk/join-family-in-uk. You will see in the eligibility requirements here (https://www.gov.uk/join-family-in-uk/eligibility) that 'family' includes engagement, and that you must get married within the 6 months from arriving in the UK, during which time your partner may not work. After you get married, he will be able to work and live in the UK on the visa, constantly extending it for the next five years.
Permanent settlement is described here (https://www.gov.uk/settle-in-the-uk/y/you-re-the-family-member-or-partner-of-a-british-citizen/no/partner/you-re-still-with-your-partner). Applying for permanent settlement is not possible without having the visa first.
As you can see, the financial requirements for the family visa are lower than for settlement, exactly so as to make it possible to first have a number of years in work, and then pass the higher bar.
I hope this information is useful and that you will achieve your life plan.
I look forward to any further questions and to your rating.
Dr Ioan-Luca Vlad