I am a Canadian citizen but my son is a resident alien in the USA and is married to an American. I would like to lend him around $120,000 interest free to pay his mortgage. Can I transfer this amount to him without any problems? Are there any implications that we should know about? Thanks for your help

You can transfer this amount into the US without concern. However, be careful about the loan part - the IRS could determine that you are not making a loan but a gift, and they could assign an assumed interest amount to the loan, which they would then say is income to you. More reading here: Loans Among Family Members at SmartMoney.com