With regard to the application for finance, your DS should start the process now (especially if your household income qualifies him for more than the basic, non-means-tested portion of the maintenance loan, as you will need to set up your own account to provide details of your income for his assessment; if your household income is definitely above the threshold, your DS should not bother to give your details to start this process, as there is no point in going through the assessment, only for him to be told what you knew all along). Details such as the university he will definitely be going to can be updated later.
Re accommodation, most universities only allow the applicant to book once they have firmed - your DS shoukd check the individual universities' websites. Some do also allow those holding their offer as insurance to request accommodation as well. Most operate the system where you submit your requests early on (and by a given deadline to be guaranteed accommodation), but the actual offer of accommodation is only made later, often only after results day.
For an offer of a place deferred to the next academic year, the applicant normally has to meet the entry requirements for the current cycle (that is, as if they were starting this September). The place would then be confirmed, but with a start date of the following year. Once the university confirms that it's requirements have been met, it can't withdraw the offer in normal circumstances.
I don't know how Liverpool organises its accommodation allocations, but for a lot of universities, June would still be within the time period for their guarantee, August less likely. He needs to investigate with the accommodation office at Liverpool what his options would be in each scenario.