yes include his pay slips from his new job, and also how many he needs from his old job to make up 6 payslips..
mention in his letter of support he was recently made redundant from first job, and has started a new job, don't forget to include a copy of his contract of employment, and if possible a letter from his manger stating how long he has worked there, what his hours of work are, what is pay is.also his most recent p60.
£2,500 is enough

, i wouldn't mention about a loan,
as long as he has about £100 a week left for you to all live off, after he's paid rent/mortgage and utils bills that's the minimum he needs.
not sure about you transferring money to his account, you could but you would have to explain in his letter of support that the money was from you (include a copy of the remit transfer from you to him, as they will notice the large deposit to his account on his bank statements. i suppose he could show he has sent you money every month (remit receipts for last 6 months? - you should be sending these anyway !! as evidence of your relationship ) , and its money you've not used, so your returning it
good luck
