Take him to eat, spend time together. I'm guessing that you go on business trips all the time, so what can you do? You're taking care of your family. It's up to you with the gifts and the amount of money that you want to spend.I would not make any changes with the amount of money that was planned on being spent on gifts though. What if you're taking a lot of business trips again this coming year? If you spend more money to make up for your absence this year, how much are you going to spend next year? Your son will remember and cherish the times spent with you more than what is bought for him.
There are so many ways to make your son feel special ... buying him 'things' is only ONE way .. not the best way, just ONE way. You want to make him feel really special .. then spend something on him other than money .. your time.