If he is on an 8MBp/s connection then that would mean near enough 800 kbp/s download speed.
You say its worse when his laptop is on, now it depends what his laptop is trying to do in terms of internet connectivity. If a torrent client is running to download files then this will take up a lot of bandwidth. That is the disadvantage to internet TV boxes, you need enough bandwidth to receive video and audio as it airs on TV.
Im not sure how effectively this content is sent to the box, but my guess is that it takes up a lot of bandwidth. Are you able to take it around to your place? Maybe you have a faster internet connection and can check whether it freezes there?