To my knowledge, Frogs are fine alone... One thing to make sure of, is that the frog you get is similar in size to your other frog... It's also always recommended to quarantine a new frog for 30 days before putting it in with your original, so as not to pass on any diseases or parasites it might have.
As to him not being very active at the moment, its more likely, since you got him recently that he's just getting used to his new environment. Moving to a new home can be stressful on a froggie. Just give him a bit of time, and he should come out of his shell

. Hope that helps!