I have been through something similar with two little ones. When I got them they had never eaten independently.
First, are you sure whatever made him sick is completely resolved? Has he been checked for parasites? Some medicines will cause a decrease in appetite (as, of course, can illness) and turning anorexia around can take work.
Make certain he is properly hydrated by daily soaks. Does he drink independently? Try a little sports drink, pedialyte, or unsweetened apple juice diluted with water.
If he was treated with antibiotics during his illness, his normal balance of gut flora will have been depleted. I use a product call Acidophiliz. There are other probiotics available as well. While I was restoring my dragons to health I used several drops daily. It seems pretty tasty to them and after a few times they would drink it off their noses. Please do note that this product needs to be refrigerated after opening so that the good germies in it stay alive. Even if antibiotics were not used, it is a good thing to do proactively.
Make absolutely sure that your lighting and temeratures are right, that the high temp area of the cage is at the upper end of the recommended scale and that the lights are bright, bright, bright. With my guys I also used every opportunity to make sure they got direct (not through glass or plastic) sun exposure for at least an hour a day. We bought a small birdhouse and set it up on the porch for their safety in outdoor basking. Also make sure that the cage environment is not overwhelming. You don't say how big he is, but if he is a wee one you may want to consider reducing his cage size, either by changing him temporarily to a smaller one, or adding a divider. Sometimes a large area is frightening. Make sure that he is someplace where he gets regular attention but not too loud or busy or with like, the cat staring at him
, and that he has a cave to retreat to if he wants (One on each end of the cage is a good idea).
I also have had success with a product called ParaZap. You can order it a number of places online. It is a tea you boil up and administer daily. It will have some success in treating any lingering parasites, and also appears to have appetite stimulation and cleansing properties. After a week of treatment with this product, along with their meal-replacement therapy (I assume that is what Insectivor is), these guys were going after food.
As sad as it is to think about, not all babies make it. He may have some congenital problem that you cannot be aware of and is just not intended by nature to survive. However, after a few weeks of this regimine, my non-eaters were eating and are getting bigger every day now. I hope you some of this information is helpful and that your dragon has a full recovery.
Best,
Michele
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4.3 Bearded Dragons (Rescues)
1.0 Corn snake (Rescue)
0.1 California King
4.5 Royal (Ball) Pythons (3 rescues)
1.1 Dogs (Rescues)
2.0 Cats (Rescues)
2.2 Humans
No, it's NOT a big house