Hi Tina!
It's a difficult one and will ask my SW consultant next week, see - if you give him too many syns in first week and he doesn't lose - then he prob will be disheartend and disolutioned with how good SW will be. If you give him too few - then as you said he may not like it.
When I started I had 6.5 stone to lose and did what they said had about 10 a day and still do and this is enough for me. Perhaps you could start him on 20 a day? See what happens? If he stops losing, then reduce syns to 15 - or similarly if he gets eager with SW and wants to speed up weight losses!
I really wouldn't limit the amount of free food he eats to compensate how many syns you give him - that defeats the purpose and "synergy" of slimming world.
Try ebay for cheap up to date SW books.
Goodluck!