Rice,
The ladies are doing a wonderful job of helping!

I'll just pop in quickly, because I believe I can shed a little light on your particular situation. My oldest son (who graduated high school last May) did a variety of math programs on his math path. He did not ever do Singapore math, as at the time he was coming up through school, there was no U.S. Edition of the Singapore math for his level. He was always ahead of the Singapore releases for that edition here in the U.S. As part of my search for the perfect math for him, I did way too much switching. We did use TT 6 and TT 7 in their entirety though, followed by No-Nonsense Algebra and then VideoText Algebra. So, I can help with that part. My next kiddos have all done Singapore all the way through as scheduled in the HOD guides, and my second oldest son (who is a sophomore) is following the HOD high school math path, using Forester's Algebra I as a freshman and Geometry: A Guided Inquiry as a sophomore this year.
Our HOD guides used to recommend the option of going into Discovering Math 7 (which was a Singapore-based product that followed Primary Mathematics). However that particular series "Discovering Math" has been discontinued, due to a copyright issue with its name. So, it is no longer sold. Prior to the copyright issue, the Discovering Math series was a good one until it was rewritten with the Common Core in mind and lost much of its Singapore-like feel in its redesign. So, we had phased out our recommendation of it with the redesign, prior to it being discontinued.
With all of that background in mind, as far as your situation goes, it is good to note that TT 7 and TT Pre-Algebra are two very similar courses. If you compare their Table of Contents, you will note this. So, it is recommended that you do one or the other, but not necessary to do both, unless you have a very young student with whom you do not wish to get to Algebra too soon.
It would be harder to make the switch to Singapore math at this juncture without going quite a ways back. It would be typical for an older student who is new to Singapore math to start around 3B or 4A and then move forward. Usually, placement testing will end up placing kiddos in one of those levels. So, if you are willing to devote time to teaching the Singapore way of thinking to your son and are willing to backtrack quite a ways to do it that is an option. You could set a goal of completing Singapore 6B with him by the time he finishes grade 8, essentially completing 3 levels in 2 years. That is very doable, but it will take your help as it will be a different way of thinking. It will be important to follow the word bubbles which show the Singapore way of thinking in the textbook, and to do the mental math mentally rather than writing everything out, and you'd have to pay attention to learning the bar method they teach for working story problems (which is a wonderful tool by the way).

In essence, you'd have to take care not to constantly default to doing math the way your son learned to do it previously. Be encouraged though that this is what all of us have to gaurd against as we help our kiddos with Singapore math, as we have to hold back just teaching them to do things they way we learned them!
A second option would be to do TT7 (or if your son found TT 6 very easy, instead do TT Pre-Algebra). After that you could head into No-Nonsense Algebra as an 8th grader followed by Foerster's Algebra I as a freshman. Or, you could begin VideoText Algebra as an 8th grader, going slowly and continuing on into his freshman year.
We would just visit again after he completed your choice of the TT7, TT Pre-Algebra, or Singapore 6A/6B and see how he did with that in order to know what would be the best next step at that juncture!

If you do choose the Singapore route be sure to give the placement test Primary Math U.S. edition linked above the math section of this page:
http://www.heartofdakota.com/hearts-for ... ckages.php
I'd begin placement testing at 3A being sure to give no help or guidance to truly see where your son places. He'll need to be able to figure out what is being asked in each problem on his own, as this is part of the daily correct placement in Singapore. If you interpret the wording on the test for him, you'll be doing it daily in the math too.

Accurate placement is huge in Singapore!
Blessings,
Carrie