That's an adult, and the symptoms suggest illness. It could be paramyxovirus but there are also several other possible causes which all produce similar symptoms (lameness or paralysis caused by infection affecting the central nervous system.) The bird can definitely survive the illness and recover, but will need help eating and drinking, possibly for several weeks. Foods (seeds, grains, and a few chopped bits of veggies) that are naturally antibacterial, antiviral, and/or antifungal would be helpful. You'll know when he is getting better because he will seem less tame and more afraid of you. If you've given him a box to sleep in, line it with paper towels for easy clean-up of the droppings. A bit of lemon juice in the drinking water (a teaspoon or so per cup) will provide electrolytes, which aren't needed after he is eating well. Occasional crumbled hard-boiled egg yolk will supply vitamins and minerals that are somewhat lacking in seed diets.