I'm sorry to hear you're having a tough time with your new little one.
I don't have any experience with very young hoglets so maybe someone else will give you some advice about the quilling and whether this would effect their appetite. How long have you had him and did you try changing his food from what the breeder/store was feeding?
With syringe feeding, here's a tutorial with some advice if you haven't already seen it.
I haven't had to syringe feed myself. Have you tried offering the babyfood mix just in a small dish and seeing if he'll just eat it himself? You could try adding kibble to it too so he's just not eating baby food. Just add some water to the kibble and let it soak then mash and mix together. Still offer his regular dry kibble in a separate dish.
If he'll eat some kibbles in his sack, leave him some at night time in his bed area. I've heard that younger hogs tend to sleep a lot, so he might be more inclined to eat if it's closer by.
Other things that could be effecting his appetite... What is the temperature of his cage? if it's too cold /on the cooler side, this can reduce their appetite. Also do you use a lighting schedule? Is it completely dark at night (approx 12 hours) and then use artificial light like a lamp or ceiling light during the daytime (approx 12 hours)? This can help get them in a routine of when they should sleep and be awake. Sunlight isn't enough as on rainy and cloudy days can make the room dark.