I started in the tree biz a year ago been climbing for ~4 months now I got 16 on hire and a $1 raise for getting my CDL so 17 per on straight time and 25.50 on anything over 40 in a week. My company provides everything for work other than my shoes, socks, and underwear and lunch.
"Close to 20/hr" on OT seems low for a climber IMO but pay for a climber varies widely based on area.
But that being said if he's only doing landscaping type work most days I can't see climber pay for running a ZTR or spreading mulch.
Seems to me like you ought to let him walk if he wants and get a climber for hire when you need one. Just my $.02