It is impossible to give you even an educated guess on the likely sentence as there is some fundamental information you have not provided.
Firstly, you mention that he has been charged with ABH and GBH section 1. Firstly he will not have been charged with both, only one or the other. Secondly there is no GBH section 1. GBH (Grievous Bodily Harm) is contrary to the Offences Against the Person Act, 1861 and falls into two sections, S18 (GBH with intent) and S20 (simple GBH). The difference in likely sentences is considerable. For GBH S18 (the most serious) custody is almost a certainty; for ABH (the least seerious) custody is rare. So before saying whether custody is likely you need to find out what he has actually been charged with (and pleaded guilty to).
I'm sure jack is right in most cases and that his solicitor should have more of a clue than anybody here on AB. However, from what you say, this may not be the case this time. It seems there is a chance that his solicitor has not given you a reasonable assessment of the likely outcome. You mention that he has been told that a fine and probation is likely. Though technically possible, a sentence involving both is unlikely in the extreme. Furthermore your solicitor can have no notion that "probation" is a likely outcome. Even if a "Community Order" is deemed suitable then supervision by the probation service (which is what he means by "probation") is only one of a range of requirements open to magistrates or a judge which fall under the Community Order umbrella. Others include curfew orders, unpaid work, and specific programme orders (such as anger management). Even if he believes that the offence attracts a Community Order he can have no idea what requirements that order will involve.
To comment on some of the other answers:
If he has already pleaded guilty whether or not there are any witnesses is immaterial. By pleading guilty he has accepted the prosecution's version of events and no witnesses will be involved.
Any negotiation around the charge will already have been done because negotiation is not possible after a guilty plea.