This just my opinion, and others may feel differently about this, but I would give the lizard eveything. He may very well have a parasite load, even with a negative on the fecal. Over the years I have found it advantageous to periodically treat wild caught collareds prophylactically. My vet recommended that I rotate giving them Panacur and Flagyl every couple of weeks if sick, less often if apparently healthy. I noticed that if I went a long time without treating them, their weight would often drop and then pick back up again after treatment.
For the very, very sick collared lizard that I am treating now, I am using Baytril, which is an antibiotic (she has an obvious infection over her eye), Panacur, and I will administer Flagyl in a week. Already after five days of treatment (along with supportive care for nutrition and hydration) I have seen a DRAMATIC improvement. We didn't even bother with the fecal since all evidence pointed to parasites.
In short, yes, give the medicines.
Make sure the lizard is well hydrated by giving water by hand, and be sure it is eating, too. I have even had to tube feed mine because she was too sick to eat at all.
élan
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0.3.0 Collareds
1.0.0 Mali Uromastix
2.1.0 Green Anole
1.0.0 Chinese Dwarf Newt
1.1.0 California Newts
1.0.0 White's Treefrog
0.0.1 Green Tree Frog
1.0.0 Fire bellied toad
2.1.0 Felines
1.2.0 Canines
1.0.0 Equine