This link describes the type of injuries which result in a Section 20 charge. (Read paragraph 55):
http://www.cps.gov.uk...on/#Unlawful_wounding
Scroll down to the next section to see what adds 'with intent' to that charge, upgrading it to 'Section 18'.
Based upon that link (in particular paragraph 65) it's hard to see how the charge could be dropped to 'Section 20'. (There might be a very slight chance of getting the charge dropped to 'Section 47, ABH'. Scroll up to paragraph 46 - but I think that such a reduction in the charge is unlikely).
The fact that the baseball bat was carried to the scene of the attack makes the offence more serious than using a weapon which came to hand at the scene. The 'starting point' sentence for "pre-meditated wounding or GBH involving the use of a weapon acquired prior to the offence and carried to the scene with specific intent to injure the victim (but not resulting in a life threatening injury or particularly grave injury)" is a prison sentence of 8 years, with judges expected to pass a sentence of between 7 and 10 years. However those sentences apply to a fist-time offender convicted after a trial. Your nephew's previous convictions for violence can push the sentencing range higher but an early guilty plea can see the sentence cut by one third.
I'd estimate that the judge would pass a sentence of around 6 years ( = 9 years less one third). If so, your nephew could be considered for release after 3 years but he would still remain in prison unless the authorities were confident that it was safe to release him. (He'd then be released after 4½ years).
Chris