Yes, your FiL should be able to register his property to complete TM30 reports online, either using the website you quoted or the Section38 phone app. I registered myself as the housemaster of my wife's house so I could do my own TM30 reports. The documents I uploaded were a copy of my wife's ID card, a copy of the blue house book for the house and a copy of my passport ID page. I got my userid and password the next day by email. Documentary requirements to register may vary depending on your immigration office. A lot of other people have done the same. Once registered, all subsequent TM30's can be submitted online.