Looks like he's soaking in the water, not just dunking his head. Check for mites.

After the vet takes samples for the culture mine have prescribed either baytril + fortaz or baytril + naxcel antibiotic combos while they wait for the results. The shots were given every 3 days for 30 days unless the culture indicated that a change was needed. Your vet will need to weigh your snake and determine the proper doses.