Yeah it is a tough spot to be in. If you really want to get him into PE, it would be precarious at best trying to lead him to the forum directly. The least damaging way is to get him to think it was his idea to look into PE. But I am not sure how to achieve this optimal outcome. You could go a roundabout way. Introduce a toy to the bedroom as foreplay or something indirect. A simple cock ring is a pretty common addition to any bedroom, but has the benefit of pushing his thickness to the max while wearing it. You don't want him to think you got it to make him bigger, but you could simply say you thought the idea of seeing him wearing one was a turn-on and wanted to see him in one. He could use it and realize that his thickness can be altered. Pumps are less common, but there are still lots of women who like to use one on their partners during foreplay. That would be trickier to implement smoothly as it is quite on the nose... You could go a more direct route and the next time he makes a joke at his own expense you could mention that if he doesn't like his size he could always try and change it and there are ways you've heard of, but you would want to reassure him that you are happy with how he is now and that it gets the job done for you. It's much more direct, so more risky. There have to be better ways of going about it, but whatever the way you go about it, the real point would be to introduce him to the idea that his thickness can be changed. He has the driving motivation already. Just a little bit of curious browsing around will usually lead to finding this site or something similar, the concept just needs to be in place in his mind.
It seems to me the best thing to do would be to try to help his self-esteem yourself in a similar way that it was injured. How you see him, or how he thinks you see him, is affected by the things you say or how you behave. Just as a slight oversight brought him negative feelings of self-competency, affirmations that you desire him and that when you have him you are satisfied can uplift how he thinks you see him. He thinks that he isn't enough for you and you're just trying not to hurt his feelings about it - Show him that he is enough, and communicate that he satisfies you. A simple positive comment about his performance or how good he left you feeling can leave him feeling mighty good about himself...