Why won't my rat let me pet him?

I've had my rats since they were less then a month old and now thye are 8 months old. One is a Rex and the other has silky hair. Their not scared of me, they like to run around and play with me, take treats and eat them near me,, ride on my shoulder, play tag or wrestle with my hands and stuff lke that. But my Rex won't let me pet him at all. If i try to touch him he just moves away and if i tried to pet his back he just shrnks under my touch.. His brother on the other hand loves being pet and will clck his teeth and boggle his eyes when i scratch behind his ears and he'll even come sit on my lap and fall asleep with me petting him. Theyve always had the same treatment and i love them equally, and my rex will climb on me and occasionaly sit on my lap but will never ever let me pet him. Any ideaS?