If there are many slides to scan (after picking the best) it may be more cost effective to DIY. Especially so if you or your father are still shooting film.
What equipment and imaging software do you currently have? What bodies, lens, tripods, light tables etc.?
I can't help you on recommending scanners.
Ricoh does make a film copier that handles Medium Format slides. You would need preferably a Full-Frame and suitable focal length (macro would be best) lens. The copier is expensive and might be hard to find outside of Japan but I think the Ricoh UK site has it listed in the store.
Another possibility is to setup a copy stand with a digital body (preferably Full-Frame), macro lens, light table and tripod.
See the linked video in the following thread for some ideas:
Digitize Negatives with Negative Lab Pro (Lightroom Plugin) - PentaxForums.com