With the breakfast, I never realised how keen my LO could be for it. I had just been giving him muesli (a baby one as it's fortified and he wouldn't have any other cereal) or toast etc he didn't *look* like it was his fav meal in any way. It was only when someone else suggested to me to try a switch around that I discovered he would eat a whole egg for breakfast and in our case this went a LONG way to balancing his nutritional needs as before he might have eaten half an egg once or twice per week. If you save some left over dinner, steamed veg for example he could have it straight from the fridge the next morning, or make an omelet. Worth a try for a week even if he still doesn't take it.
Teething goes on and on and ON. The molars especially are awful, I thought canines were bad until molars started. And yes as long as a piece of string, we've been having molar teething for over a year now! He's just got his last one a few weeks back but continued to have pain after it looked fully through so there must have been some final movement going on. The discomfort tends to come in waves and each one puts them off their food and can cause sleep problems, then they seem to make up for the lost calories and eat the fridge clean.
I know what you mean about not wanting to eat so much in hot weather, me too. I think though if it has been this extreme for 2 weeks, and given his age, I would prob drop in the the GP and ask for a referral. it will take time to get the appointment through and if he has a sudden turn around and starts eating you can always cancel the appointment. If he continues taking only milk you will at least know the appointment is coming up rather than waiting another several weeks yk? That's what I'd do.
I wonder if there are any times he just wants water? I assume you've tried water in place of milk for some of the oz?
Is he taking the milk in bottles or a cup? I wonder if you offered a sippy or open cup of milk alongside a small meal/snack he might go for the solids instead of the milk? Perhaps some very tempting food such as pancakes or mini muffins? (home made can be very healthy, no sugar, and incorporating fruit and veg too so it seems like a sweet treat but actually it isn't bad at all) If I was in this position I'd give him any food he would accept, even if it was 'cake' (home made with veg and fruit) just to get the solids habit going again, then gradually increase the variety of foods. Will he take sweet fruits? melon, cherries, sultanas, maybe something like canned peaches (in juice not syrup) or pineapple?
Would he and his sister find it fun to have something to dip maybe? Carrot batons and pitta bread fingers with a guacamole or humus dip for example? Dress up the food a bit so it looks visually appetising, more like a game than a meal? I'm not a fan of making food dressing up food for kids, I think they 'should' be encouraged to have family meals, but this is a different case, yk, where anything that helps would be worth a shot.
How old is his sister? Is it possible to talk with her privately about her own nutrition to discourage her from copying him with the milk?