Can a parent buy his 18-20 year old son a handgun in Texas?

My younger brother, who is 19, was looking into the new Ruger SR1911. I know he is not old enough to buy the gun himself, but could he be with my dad when my dad purchases it? Could it be registered under my brother's name? and would he be legally able to own it? Also, what is the age restriction on buying ammunition in Texas?