Hi mollysw,
I’m not sure I can “convince” anyone of anything or if I would even want to. My goal is to “educate” more than “convince” so that the “best choice” for each individual person will be clear and self evident based on their own personal value equation. In other words … the goal here is to help people learn “how” to choose rather than “what” to choose so that no matter what they buy they will have reasonable expectations and can make an informed choice.
What I would suggest is encouraging him to at least read the tutorial post which has all the basic information, steps, and guidelines that can help each person make the best possible choice … and learn how to avoid the worst ones.
Outside of making sure a mattress is a good match in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences), the quality, durability, and useful life of a mattress depends entirely on the quality of its construction and the materials inside it. The “brand” of a mattress is just a name on a label and has nothing to do with the quality of the mattress or the materials inside it. In terms of the quality of the materials … the Select Foam mattresses (and some of the others that are also listed on the site) use the same or better quality materials as the equivalent Tempurpedic mattresses they use as a reference point (or the other major brands) and are lower in price which to me would make them a better “value” choice but each person may have very different ideas of what they are willing to pay for. To me the “cost” of a major brand label sewn onto a mattress in terms of quality or value is much too high.
I would suggest avoiding any major brands or for that matter any mattress that uses lower quality materials or where you are making a “blind” purchase which is a great recipe for having to repeat the mattress shopping experience much too quickly … typically 3 - 7 years … and sometimes much sooner. I would also make sure that you don’t count on a warranty as a way to indicate how long a mattress will last (see post #174 here) or have any expectations that a mattress that has lost its comfort or support and needs replacing in a few years (or sooner) as the result of the softening or breakdown of lower quality materials will be replaced under warranty. Unfortunately … as you will read in the tutorial post … there are many people who believe more in “marketing stories” than in meaningful or factual information but eventually they will come to recognize (usually after the same cycle repeats a few times) that there is no substitute for for some good research when it comes to making a major purchase which in this case is one of the most important purchases you will make in the next decade or so in terms of the quality of your sleep and overall well being.
You can read a little more about the Radiance and some of the other Sealy Optimum memory foam line in post #48 here.
Hopefully he will decide to spend a couple of hours learning about how to make the best possible choice (and of course if he has any specific questions you are welcome to post them here) and then if you are both on the same page, the rest of your mattress shopping experience will be much easier and you can combine your new found knowledge and energies instead of working against each other.
IMO … foresight is always a better approach than “buyers remorse” or the regret that can come with hindsight 
Phoenix