Sounds like a goodplan for both of you especially if you don't really need him for meat, at 26 weeks he may be a little young to work, working boars lives nornally start at around 8 or 9 months. the only difficulty you may have is his size, he may be a wee bit too small to serve the sows especially if they are mature i.e over a year old. if they're young gilts he may be ok.
Normally i advocate going down the pedigree track as it saves a lot of grief in the long run but if this deal works for both of you then fine, work out what he's worth to you as meat and then ask for the number of weaners (£40 each normally) to cover that.
HTH
Mandy