I was just reading how you said that your child bites himself. My son is now 5 years old and when he was 2 he was biting himself everytime he got angry with himself or someone or if his shirt was bugging him. We went through that for a year and 1/2. My son also is one of the most polite child you would ever meet. We were lucky that when he start day care that there was a Preschool conselor on site. The first day my son started they wanted to take him on. They worked with us by giving us ways to deal with him. They even gave guidelines to the Teachers on how to work with him. Our child was diagnosed with ADD. I know it might not be the same situation but eventually we had no choice to put him on meds. It hurt at first to hear that because being that I used to be an Educator in a Day Care and always told Parents to never consider meds unless it is the last resort. Now my worst fears were coming true. To tell you the truth it was the best decision that my husband and I could of made. Along with teachers help, he has not bit himself since. Don't get me wrong he still has his outburst by maybe screaming, throwing things. But our goal is to teach him to control his own anger and hopefully to get him off the meds. He is more enjoyable to be with. I might not of helped you but I wanted to tell my situation with the bitting.
It sounds like your son has a number of significant stengths against a backdrop of (a) difficulty with frustration and (b) delayed social development with peers. While he can probably manage fine without professional assistance, therapy might afford him the opportunity to develop better strategies to handle frustrating events (he will, after all, face such events throughout his life) and to address his reticence around intercating socially with peers. Use of an incentive system is a good strategy to support him in managing frustration. Be sure to employ rewards that have some meaning to him. For example, if he likes to play video games, have his use of such games contingent, on a day-by-day basis, on avoiding the episodes you describe as 'losing it'.