Look at this link
https://www.gov.uk/jury-service/what-you-can-claim
£64.95 a day plus travel and food cost paid , you can claim child care cost as well.
I would add that just because he is summoned dos not mean he will have to actually go. They summons a lot more people than they actually need as there are a lot who drop out or can not serve. Another tme I was summoned I had to phone in at 4pm every day to see if I was going to be needed , it the end I did not have to go.
Not really the 'right thing to do' but anyone who knows the person on trial or anyone , family etc, connected with them can not serve on the jury. He could phone up and say 'sorry, but I know the defendant' that would get him out of it. Obviuosly you can't do this until you know which trial you are on. I know it happens a lot