Sacked without warning advice required

Hi I would be grateful for any advice on the following. My Son has been employed for 18 months, his role changed however he was never given an updated job description. A number of other employees were dismissed and he had to take on their workload as well as his own, it is common knowledge within the office of his many verbal requests to his Manager for help and support. He has also requested performance reviews which he has never had. He has also made a small number of requests via email to his Manager, (he does no have copies of these). Today he was asked to go into the office by his Manager when he asked why he advised it was for a performance review, when he entered the room another Manager from a different dept was in the room, he was handed a letter to open, he opened the letter and it was from HR. His line Manager then advised the letter was for him to attend a meeting tomorrow due to work performance, basically not completing work in required timescales. I am unsure if he requested a member of staff to be included in the meeting or if it was suggested (colleague joined the meeting). My Son stated he had made numerous request verbally and via email over the last few months for help and additional support to which no response was given just a shrug of the shoulders. My Son asked if he was being sacked and requested the meeting request for tomorrow be brought forward, He was then dismissed and walked out of the premises by the Manager from the other Dept. This Manager apologised to my Son and asked if he was happy with how the meeting had gone to which he replied no he wasnt, said Manager then advised there are things you can do about it twice but did not say anything else. My Son immediately try to access his emails remotely to obtain his requests for help and support but his access had already been blocked. They are paying him 4 weeks notice. Can some please advise if he has any chance of taking this further, apologies for the length of this thread but wanted to ensure I covered everything,

Sacked without warning advice required