Hi!
At this age and weight, I probably wouldn't try to change his food just yet. But if you are feeding kibble, trying cracking it so that he doesn't have to cracke it himself to chew. It really shouldn't be a problem for him, but perhaps it is. Or you can wet the food a little to make it softer. Remember he is really tiny and doesn't have room to eat a lot at one time, do you leave food out available to him at all times? I free feed mine and would see Gracie Ruth and Yogi stopping by the dish several times a day to grab a bite or two. They'd eat their main meals in the morning and evening when Boo would eat, but they'd snack on little bites all throughout the day.
I also would stop giving the Nutrical on a regular basis. It really is meant to be used in an urgent low blood sugar situation. The Nutrical can cause some to not want to eat. It seems to make their systems not feel hungry so they don't eat.
Have he been to your vet for his puppy check up? You can always bring up your concern when you go in for his next round of shots.