Is there an English (British or American) expression or idiom that refers to a recluse finally socializing

I'm a fan of "emerged from his/her cave." It seems to fit the concept described in the updated version of your question, of someone who is making a rare foray into society, rather than entering society on a permanent and ongoing basis.

EDIT: Changing my original answer in view of the OP's updated and revised question.

He's emerging from his den/shell/cave/burrow

Seem to be the most appropriate, I asked a few native speakers and nearly all came up with "emerging from his + (noun)" as first suggested by user867.

Google Ngram seems to suggest that emerging from his den is the least commonly used expression, but if you click on the actual results you'll find that the "emerging from his den" is commonly used for bears and people, whereas emerging from his shell can also refer to tortoises, turtles and snails, so depending on the person's physical traits you could opt for either.

There is a word for it too - reclaim

reclaim v.3 To save (a person) from an undesirable state, course of action, etc.; (also) to bring back or restore (a person) to a better or more acceptable way of life or condition. OED has many example sentences:

A young man, who was reclaimed from a very dissolute course of life.

Here is Calvin, reclaimed—from substance abuse, from financial uncertainty—so strong now.

The Pink Floyd song comes to mind coming back to life. The closest English parallel to your phrase is breath of fresh air. Although this is not what you are looking for, a closely related phrase breathe new life into might be relevant.