Has this actually been officially confirmed. Just because he seems to be wearing something of theirs surely does not mean he has a contract with them.
Nothing has been officially confirmed, and according to what I have read neither the people from Lagardere who work with Andy's management to negotiate etc the business side of things for Andy, or Under Armour, have provided comment either way on this.
However today two Baltimore (UA is based there) newspapers published articles about a rumoured deal between Andy and UA. Links below, but the interesting things are: the deal might be similar to the Adidas deal but offer a combination of stock and cash, and UA are developing a tennis shoe. UA seem to pride themselves on innovation and development - that was the point of the company in the first place - so they are likely to be keen to develop new products, ideas etc with Andy as the face of the innovation. They might be interested in working with new things like wearable technology, which Andy has expressed an interest in. My sense is that they could be more willing to work with Andy on things, rather than go 'this is your kit', which is what Adidas has felt like for the last year or so, if not longer.
Baltimore Business Journalhttp://www.bizjournals.com/baltimore/news/2014/12/16/under-armour-nears-deal-with-tennis-star-andy.html