As GB says before WW1 they would destroy the service records of men dying in service 20 years later. Also, most of the surviving records came from old pension files. From memory you had to serve two periods of ten years service to qualify for a pension in this period – it changes often during the 19th Century. If he left after ten years they again would not have kept his record.
The best way to proceed is to follow his service using the muster books in Kew. They are not online. I can however tell you that George Brain, regimental number 497, enlisted on 7th February 1860 in Dover. He was 18 years old, 5 foot 5.5 inches tall and a ‘volunteer from the Oxford Militia’. He received a £3 bounty and his ‘bringer’ – the man who recruited him – got 14s 6d. As a Private he earned one shilling a day.
The 2nd Battalion was only reformed in December 1859 when they decided to add second battalions for all single- battalion regiments up to the 25th Foot. They got large numbers of recruits for the Regulars from the Militia. In this period the Militia came under the Lord Lt of the county, so you should ask Oxford Record Office to see what they have.
The 25th Regiment were in Fulwood barracks, Preston until Qtr 2 1860, when they moved to Aldershot. Fulwood survives to this day -
http://www.lancashireinfantrymuseum.org.uk/fulwoodbarracks/In August 1861 the 25th moved to Shorncliffe. In May 1862 they were split between Edinburgh and Gosport, and sailed to Ceylon in July 1863. In December 1867 they moved to India. While the service companies were overseas they would have kept a Depot in the UK. For the 1871 Census the 2nd Bn Depot was with the 1st Battalion in Gosport.
You have to be aware that men enlisted in the regiment and not the battalion, so he may have been transferred to the 1st Battalion at some point. The muster books were also the paylists so the Paymaster will have recorded what happened to him when he leaves the 2nd Bn paylist. This could be because he deserted, died, was discharged having been sent to prison, given a medical discharge, purchased his discharge, moved to the 1st Bn, or transferred to another regiment.
Ken