276L or 276O
ILR may granted after 4 years in the Army, after which he may apply for naturalization 12 months after the grant.
Alternatively, to address your question about long residence. Exempt leave can be included into the 10 year period but not while it is still on-going. So he can't apply while serving. He would have to wait until he exempt period ended and then apply.
Please also note that a person who applies for leave while still exempt should be told that there is no power to grant them leave but that if they apply when they cease to be exempt, their application will be considered.