Well, assuming you had his mortgage info, logistically you could make a payment on his mortgage. Tax liability would be a question for a tax advisor. Offhand, if you weren’t planning on deducting the contribution on your taxes, and you paid the money directly on his mortgage, my hunch is that it likely wouldn’t create a tax liability…..however, the answer might depend on who you ask (IRS or tax preparer). Not sure how IRS would find out if you just paid the money on his mortgage though.
Possible, yes definitely. The payment could be made directly to the lender as long as you have the mortgage number and proper mailing address.
As to a tax situation, I agree with Ted here on recommending seeking professional advice from the IRS or a tax preparer, but in my experience the Feds allow an exception of $14,000 per person annually. Anything less than this and there is no gift tax. And usually it is the donor that pays the gift tax. (Here is the IRS reference: http://www.irs.gov/Businesses/Small-Businesses-&-Self-Employed/Frequently-Asked-Questions-on-Gift-Taxes).