My fiance earns 60pounds a week from his 1 job and 35pounds from his other job. He doesn't have P60 because he told me that his not earning that much to pay tax

Someone's either being economical with the truth, or just doesn't understand.
If you work (officially) in the UK, then at the end of the tax year, you get a P60.
Granted, you may not earn enough to pay tax.... but you still get a P60.

He's therefore being paid "under the counter", ie cash-in-hand.
Which is basically illegal ...... as it's defrauding the tax man (HM Customs & Revenue).
... and why his employer has given him a letter stating this is beyond me. He's basically declaring his guilt, and inviting a visit from HM Customs & Revenue.

... what does he do for £60 per week ..... a newspaper round ???

.... so, the same problem as I stated before arises... he's essentially UNEMPLOYED.
You are not going to get a fiance visa in this scenario.

BTW ... why isn't he claiming Job Seekers Allowance, or other DHSS benefits, especially as his job is "undeclared", and he's unemployed ?