He is either suggestible or not. He can't only be suggestible when it benefits the defense.
People in the "borderline retarded" IQ range are suggestible when it comes to a questioning situation because they are seeking to escape from the questioning. Therefore, they will often repeat what is suggested to them by their interrogators. However, that does not necessarily transfer into actions, i.e., following another person into an illegal activity.
The suggestible nature of those with low IQs is simply a defense mechanism to allow them to escape unwanted (or uncomfortable) interrogation. If another person wanted to pressure one with a low IQ into doing something illegal (or immoral), it would have to be done through intense questioning, not through merely asking or suggesting. It's kind of like saying that hypnotizing someone can make them murder. All hypnotists point out that you cannot be forced to do something against your moral code under hypnosis.
It must be pointed out here that Jessie and Damien were not "friends." In fact, Jessie knew Jason when they were young, but Jessie and Damien were merely acquaintances, through Jason. So, I seriously doubt that Damien could pressure Jessie into doing something. Jason might have had more influence on Jessie, but I don't think Jason would have pressured Jessie into murdering those three little boys.
Jessie's friends will still have a great deal of influence on his perception of "right" and "wrong" however, but this is true of many of us with "normal" IQs. However, I don't think Jason pressured Jessie into murder as I don't think Jason himself is capable of murder. But, hypothetically, if Jason had wanted to exert such pressure, he
might have been successful.
(I recommend reading the GQ article if you haven't, especially the part where Jason talks about the prison personnel that rejoiced upon his release. They recognized that he doesn't have the temperament of a murderer. Therefore, he would not have tried to convince Jessie to participate in murder.)
Jessie wouldn't have thought up the murder plot himself nor would he have been unduly influenced by Damien. So, his suggestibility is only applicable to the statement obtained through intense interrogation, not to actions taken at the possible suggestions of Damien. Again,this is a direct result of his low IQ and his reaction to questioning as a result of that IQ.