How long has this site been doing 20k a year? How much is his help worth to you?
If I were to provide support for something like this, which I assume will include phone support, I would probably estimate about $30/hr so 6 months times 4 weeks a month time 2 hours a week comes to about 24 hours worth of support.
If the domain is decent and the site has been doing 20k a month consistently for 2 years or more then I would offer him about 12-15 x monthly revenue. The 10x factor I believe is for relatively newer sites, ie < 1 year old.