I don't know if this will help at all or even if it's relevant but I find that for difficult eaters (I don't know if he is?), leaving food out all day is likely to make them eat less, as they will never really be hungry enough to eat a full meal if they are grazing all day. I would say say feed him as normal, just add a bit more food to each meal, perhaps also add some food higher in calorie density, and try not to treat him too much during the day to ensure he eats his meals properly.
Also, when I left one of mine at a friends for 2 weeks, so not even a kennel, she ignored me for 2 whole days afterwards and was acting so strangely I thought she was actually sick! I realised that I was babying her and that's why she was playing it up. So I started to act normally again and eventually she forgot she was peeved and went back to normal as well