Take him to several stores to play guitars before you spend any money! It would be such a waste if you dropped $1500 on a guitar that he doesn't enjoy. Honestly though, take him out just to "look" at guitars, he can dream about amazing guitars and you get the exact research you need, the easy way by chilling with your dad. Make a day of it and hit up all of the best shops in your area. Do you know exactly what your dad is looking for in a guitar? Find out exactly what body style will fit your dad's playing style, which tonewoods he prefers for his playing style, both back/side woods and top woods. For every serious player, there is a guitar on a wall somewhere that will scream "Take me home!" as soon as they play it. It's just a matter of finding that guitar...for some people they never find it because they don't look.
Since you asked: Taylor vs. Martin
Taylor guitars are overpriced pieces of shit that produce less tone than most other acoustics I've played. The only guitars that put out less "sound" than Taylor Guitars in my personal yet experienced opinion are Ovation guitars. Taylor guitars use a bolt on neck instead of the traditional all wood dovetail neck/body joint. Think of it this way, if an acoustic guitar's only job is to transmit the vibrations of the strings into beautiful sounds, why would you incorporate metal into the most important connection of the acoustic guitar (neck and body joint)? The answer is, you wouldn't if you had a fucking clue. Yes, a lot of guitar makers are going to bolt on necks for acoustic guitars now, but only because they are easier to make adjustments to the neck instead of performing labor intensive neck resets.
Martin has been in the game for a long long time and still builds pretty decent guitars (again, in my experienced and very biased opinion.) I would definitely make sure you are getting solid back and side tonewoods when dropping around $1000-$1500 for a Martin, as they do use laminates and HPL (high pressure laminates) frequently. Martin does still build guitars in the traditional fashion and does use the dovetail neck/body joint that any purist would require of their instrument.
As mentioned above, Larrivee does make extremely nice guitars, and they can be had for around $1000. I would much prefer a Larrivee over a Taylor, or Martin any day. There are tons of companies worth checking out though, that is my main point. Don't limit yourself to just Taylor or Martin.
For what it's worth, I have been playing guitar for over 16 years, I have a degree in Guitar Repair and Construction and have built my own instruments. My opinions are just that, but I have been fortunate enough to have a hands on approach to most of the guitars on the market and have gone out of my way to play as many guitars as possible. I have also been fortunate enough to have been a part of seminars put on by major guitar manufacturers that shed more light on their instruments and manufacturing processes than a typical consumer could ever learn. If you have any questions regarding things with six strings, feel free to hit me up with questions any time.