Hi Alexh, I wouldn't be too concerned about protein all the time he's drinking milk but if you're worried theres a few things you can try.
Firstly, you could try making up a batch of chicken or beef and potato puree and freezing it in ice cube trays and just add one to his favorite meal for a few days so he can hardly taste it, the after a few days put in 2 cubes, and gradually increase the ammount he's getting so it's introduced slowly and he'll barely notice.
If that's not an option cheese sauce is a good one, I made a batch of veg puree in cheese sauce and gave a few cubes of that per meal with a chicken and potato puree. I've a cheese sauce recipie if you want it.
You can get low sodium baked beans, these mashed up with mashed potato go down well, I've found.
Cheesey toast fingers if he's feeding himself.
If none of that appeals you could get a bag of pulses and pearl barley and try him on veg stew with pulses. Lots of protein in them.