azalea82 has a good point. An unsafe way to treat his problems is to blindly start giving him stuff. If he has autism, then how he is treated by school personnel, parents, and peers is as important or perhaps more so then what medication he is given. His teacher needs to be given sound advice which should be written into your child's IEP. If he is not in special education, then he needs to be - it is the only thing that can force a teacher to treat him correctly. The most important thing is to find (if you have not done so) a good pediatric psychiatrist and listen to them. Amazing things are being done with Autistic kids - if they are treated correctly.
L-theanine is a supplement and is therefore not regulated by the FDA. In this respect, it may be less safe than prescription medications. Perhaps an evaluation by a pediatric psychiatrist would be of benefit. The medications that doctors prescribe must meet standards for safety and purity, while supplements do not have these requirements.