The offer on the foundation year, which is effectively an Access year at university, the difference being it is tailored towards the needs of the follow on course. So if your husband has a change of heart about where to complete his degree, he may have fewer options.
University applications are down this year, most 18 year olds do not apply for foundation courses because the already have sufficient qualifications to start a degree. Entrance qualifications are stricter for under 21 year olds. Universities are allowed to waive level 3 qualifications for over 21 year olds with relevant work life experience. It sounds like your husband's CV was sufficient. Universities can also accept direct applications, they choose to use UCAS for their conventional recruitment because it allows the have a better idea of how many students are likely to take up the place. Before UCAS or it's predecessor UCCA, a university could easily end up oversubscribed or undersubscribed, they would only find out on enrollment day.
Have the university made your husband a formal offer in writing? If they have then subject to him meeting the offer criteria, his place should be secure. If they are offering him place at this stage, they have space for clearing. If they haven't confirmed it in writing, get them to do so, then if your husband gets his Maths GCSE, he will get the place. Even if you put in a UCAS application in now, it would not be sent to the university until clearing opens after the A level results.
For peace of mind, get the application form now, fill it out and submit it with everything but the GCSE result. When your husband gets his results, submit the result with a copy of the application.
Someone I know made a late application, the week before the course started and secured a place on the course they wanted at university. They just made a phone call to confirm they had places available and a check to see if their qualications were acceptable. Universities can cope late with late applications if they want to. What they are asking you to do, is not dissimilar to the process used for clearing, except you not UCAS seHnds the application through.
By the sound of it, you still have the insurance policy of the Access course place, so don't give that up until the final confirmation comes through from the university.
Out of interest, how much is the university charging for the foundation year, as compared with the Access course.