My twins are breastfed so no nasty formula luckily. That's why it wasn't until starting solids and DS having a bit of cheese that we found out about the allergies.
It does mean I have to be off dairy too though
Very good for my waistline though - I'd recommend bfing twins and a dairy free diet as a very effective weightloss regime!
As for sleeping through the night, don't ask me - mine don't! I don't think in their case it's linked to food any more though, as they eat plenty, but the sleep is still pretty rubbish. They share a room and disturb each other which doesn't help, but there's not much we can do about that.
I think in your situation I'd accept I can't get much formula in, and try to pack as many vitamins and calories into the solids that he will eat - tricky but not impossible when he's dairy free as that's an obvious milk replacer and source of calories.
Remind me what foods he can't have? Dairy? Wheat?
Avocado is the highest calorie fruit I believe, and mashes with banana into a nice puree or sandwich filling.
Add olive oil to everything you can, like mashed potato etc, to up the calorie content.
Protein and complex carbs take longer to digest so think about using these if you want to fill him up for longer.
Soya milk (not organic) is a good source of calcium and vits and is fine for making cereal, white sauces etc. Something like porridge or weetabix made with soya milk should fill him up pretty well. Mine like the Alpro soya yoghurts too, though Asda's own brand ones taste a bit odd to me.
There's Oatly oat milk too, which he might prefer.
Once's he's onto finger food, Soreen's banana loaf is egg and dairy free, though I'm not sure about gluten as that's not an issue for me.
Maybe try making a smoothie with soya milk, banana and strawberries - would he drink that?
Speak to your HV about vitamin drops. They should be able to supply them very cheaply and you can mix them in with his food.
I always find this stage of weaning stressful though, and I've done it 4 times! One day they like something, the next day they don't. One day they want finger food and won't be spoon fed, the next it's the other way round.
Things will have settled down a lot by about 9 months, and I'm sure his intake will have increased. Keep offering a variety of foods, and finger food too, and he'll get there.
Hang on in there, sleep deprivation is tough, weaning is tough, weaning with allergies is extra tough but he will get there, honestly 