Did you quarantine your new arrivals for at least 30 days prior to introducing them as tank mates? My hunch is that something new was introduced into the environment that is causing him to become sick.. Looks like a bacterial or fungal infection. Those spots should be treated asap, start with a non-pain reliever neosporin. apply a little dab to the spots twice per day with a clean q-tip when the frog is sleeping. Be sure not to cover the whole of the frog because he actually breaths and drinks through his skin and the neosporin can suffocate the skin.
My recommendation, Take to a vet as soon as you can or email Dr. Frye:
734-439-CARE (2273)
(don't have his email off hand, but you can call his office, they are so nice)
Good luck