Generally speaking, no -- feeding Gro-N-Win should not affect his stifles in a bad way. That being said, a horse's nutritional affects EVERYTHING about the way his body works. I would be more concerned if you said you feed a handful of All-Stock and yard clippings -- then I would worry about his muscular, ligament and connective tissue development from lack of protein and other nutrients.
Basically, luxation of the patella is a conformation issue and is a result of less than perfect alignment of the joint and its attendant ligamental attachments. Think about a person who has a "wandering eye", where one eye might turn out or turn inward a bit. The attachments that hold the eyeball straight are either too short or too long on one side or the other, so the eye "wanders" rather than remains in the looking straight ahead position. This condition (known as strabismus) is not related to nutrition, but rather to "conformation" of the eyeball and its attachments.
Sometimes it is just a growth issue and given time, the horse will work himself out of the condition. You didn't mention how old this horse was.
Surgery is an option, but before I would resort to that I would pay close attention to hoof trimming and exercise. I find that my "stifly" horses do best when trimmed very frequently, even as much as every 4 weeks. Let those hind toes get long, and it's a sure fire way to get them "sticking". Regular exercise may help strengthen the tendons/ligaments/muscles as well. Another consideration before surgery would be to try a supplement such as Corta-Flx liquid with HA (hyaluronic acid). I've been very happy with the results of this glucosamine/chondroitin/MSM/HA combo in my "sticky" horses. Gosh forbid if I run out and don't give it to them for a few days, because sure enough, we're back to sticky stifles. 3 days after putting them back on it, they're good to go. Doesn't work in every case, but it's worth the $38 for a bottle as a trial - cheaper than surgery!