Has his name been added to the title?  If he's on the mortgage now, it sounds like he would have an economic interest in the property as an equity owner, provided he does, indeed, pays down the mortgage (and is responsible for other expenses related to the rental and maintenance of the property).

In order to determine whether there was or the extent to which there was a taxable gift on the encumbered property, I'd ask a couple more questions.

1. Does the mother intend for 50% of the property to be gifted to the son?
2. What was the mother's adjusted basis on the property?
3. What was the property's FMV?
4. What was the amount of debt assumed by the son?

The above would then establish the son's basis in the rental property.