This is probably not recommended, but salt added to food makes it tastier and he might eat?
Please check with a dietician before you start putting salt on his food.
My dd was on the bottom line of weight charts all along so she was under a dietician and a paediatrician.
Protein is the one I'd be most concerned about. My dd was prescribed a powdered calorie thing for me to add to her food. I was also advised to add butter!!!
Yoghurts such as petit filous are ok but I was advised not to give them by the nurse but then advised to give them by the dietician.
The other thing is all of you eating at the table together. Put him on your knee or on his Dad's knee and pretend nothing unusual is going on. He might try to eat off your plate. Don't necessarily offer him any. This is going to sound gross, but try sharing food with your husband across the table and he might want some too.
That's the only advice I can give from my very personal and limited experience.
In terms of chicken nuggets, you can make your own which will be infinitely healthier than anything you buy. I buy these from Lidl/Tesco etc. mini chicken fillets
To cook them, you can buy breadcrumbs (yes, it's powdered crap). So you lay out a baking tray and heat your oven to 170/180 celcius (fan). While oven is heating up, you put out three plates. One will have the raw mini chicken fillets, the next will have either olive oil or whipped egg and the final one will have breadcrumbs. You literally take each mini fillet, roll it in either the egg or olive oil and then roll it in the breadcrumbs. Place them all on the baking tray and about 15 minutes in, check them and turn them. They will be actual real proper chicken. I love lemon juice on them (the bottled lemon juice is fine) while they're cooking.
You'll have a batch of real chicken cooked - home made goujons! A grain of salt won't go astray. Don't put pepper or spices on.