ETA - on another tangent - I have often thought to myself how mad we all are having our well-rounded dinners at dinner time. Surely it makes sense to provide that food earlier in the day when your body is actually active and using it?? Do you think your son might cope a bit better having a larger (ie non-sandwich) lunch and then having a smaller light-type meal at night?
I agree we try to eat at lunch time (taking cooked meals to work etc.) and have lighter tea. We lived in Spain and this was normal and agreed with us far more - DH especially struggles with eating main meal at night finds he doesn't sleep as well. It is one of the reasons why I wish schools did school dinners rather than taking lunches in - I grew up with school dinners and wish they were available here more.