Assistive Listening Devices and Unilateral Hearing Loss
First, I must commend this parent for being so knowledgeable and seeking out help for their son. The failure rate for children with untreated unilateral hearing loss is about 35%. The research does show that CROS hearing aids will not help in the classroom environment nearly as much as a personal or sound field FM system. Concerning which is better - the personal or sound field system, there isn't much research. Certainly, the personal system would offer a much better signal-to-noise ratio (SNR) so I personally would start there if the child would successfully wear it. The SF system would be my second choice, but I would strongly consider the child's feelings in this matter.
Hope that helps.