I would be tempted to say that the axie looks skinny, but other than that, looks in reasonably good shape.
Okay, the gills aren't too prominent (yet?), but I can't see any signs of disease or particular stress (unless the tail is curled more than not).
A good diet of earthworm and maybe the odd waxworm as a treat should gently put a bit of 'chub' on him.
When viewed from above, a healthy guide-line is if the width of the head equals the width of the 'belly', more or less. Gaining weight slowly and steadily is healthier than reaching their ideal weight straight away, of course.
Now probably the biggest risk to your axies health is the gravel that you have as substrate. (This is a very easy mistake as pet-shops and certain web-sites don't alert people to the problem of gravel)
Follow Ersz's excellent advice and you won't go far wrong, and I agree with bayhicoach, it should be in full health before too long