Using GAI as a more accurate measure of ability is pretty common, particularly since lower WMI and PSI scores are extremely prevalent amongst gifted kids. (A 124 on WMI is above average, by the way.) A GAI of 133 is an excellent score and should be enough to qualify him for most gifted programs.
I'm not sure why you seem concerned with him not being "off the charts." If anything, this should make your advocacy needs less intense.
Does the school have a gifted coordinator or advisor to his K classroom? Sitting down with them and his teacher to have a proactive conversation is a good idea. Find out if they do in-classroom differentiation and what exactly that means. Then, as the year progresses, ask your son if he feels like he is learning and being challenged in class. (Ours flat out told us that he wasn't learning anything, even with differentiation.)
There shouldn't be a reason to keep WPPSI results to yourself, as the test is well respected by most institutions and should carry enough weight to grab the attention of educators. Plus, giftedness can have many different faces is often not spotted by teachers (they have plenty else to worry about)... so if it calls attention to special needs for your child, so much the better.
