Yes. Moreover, if they have evidence of living together akin to marriage for at least two years, then he may apply for leave to remain as an unmarried partner, subject to Appendix FM and FM-SE.
Not sure about adoption.
Yes, his earnings may be included to help with the financial requirements under 1
(c) The employment or self employment income of an applicant will be taken into account if they are in the UK, aged 18 years or over and working legally, and prospective employment income will not be taken into account (except that of an applicant’s partner or parent’s partner who is returning to employment or self-employment in the UK at paragraphs E-ECP.3.2.(a) and E-ECC.2.2.(a) of Appendix FM, or where paragraph 21A of this Appendix so permits).