Hi there here are a few books he will like.
DIESEL TUGS A colour portfolio by David L Williams & Richard de Kerbrech ISBN 0-7110-3117-7
LOOKING BACK at BRITISH TUGS by Andrew Wiltshire ISBN 978-1-902953-28-1
STEAM TUGS A Colour Portfolio by David L Williams ISBN 0-7110-2857-5
TUGS in Colour U K by Bernard McCall ISBN 1-902953-11-8
Thames Ship Towage 1933-1992 by J E Reynolds ISBN 1-85821-028-3
and a really interesting one Take The Strain by Tim Nicholson the story of Alexandra Towing ISBN o-9516495-0-7
Hope this helps Geo.