It could easily be MBD, depending on what dust u are using on the circkets. Most of the time MBD is not cuased by a lack of calcium in the diet but rather a lack of vit. D and a bad ratio of phosporus to calcium. In order for reptiles to absorb calcium they need vit D which is produced in the body when reptiles are exposed to UVB/UVA rays. My first suggestion would be take him back to the vet and have them look at it. I can't say for sure that it is MBD because i can't touch and examin your tegu. Home remadies included getting liquid calcium which most on the market have a complete calcium meaning it has the vit D and everything already in it. For long term benifits get a UVB/UVA bulb. If you have one and its a floecant check to see what type it is. if its a 2.0 then it should be at the max 10" away from your tegu to be affective, if its a 5.0 the distance is 12" for a 8.0 again 12" and you can get a 10.0 but even then its only good for up to 15". Also remember that screen tops do filter out some of the UV.

Good luck with your tegu and i hope he gets better.
-----
Leo Gex
1.0 Giants
0.1 Red strip/hypo
1.0 Patternless
0.2 Normals
Others:
1.0 Giant plated lizard
1.0 Blue Tegu
1.1 dogs
2:0 chinchillas