Here is my newly acquired Erythrinus erythrinus.. it's a little over 4". I must say I was hoping it would be more active but my common wolf is more active then this little guy
Typically they are more active than Malas. I'd just be happy that your mala is the exception to the rule. But, the bigger and fatter he gets, the more lazy he'll get. The ery ery will pretty much be just as active as he is now. Oh, and I'm sure it's also still getting used to the tank. Give him some time and he'll come out of his shell more.