sairish, I don't know if the criminal justice system works in Northern Ireland like it does in England, but if it does, it'll go something like this...
You can get legal advice for him in advance, but you don't need to, for a number of reasons. A 'duty solicitor' will be available at the police station, and anyone you hired in advance wouldn't get any more information about the case in advance, because there is none available until your DS has been arrested; they could just give you/your DS info about options and likely outcomes.
The fact of the offence isn't in dispute - he's admitted it - and if he continues to tell the truth (always a good idea), it's just a matter of 'taking his punishment', so to speak. A solicitor will help your son decide what to do at each stage - e.g. admit guilt, accept a caution, take the matter to court, etc...
You will turn up at the police station, and because of his age, you should be taken into a private room away from adult offenders very quickly. He will be formally arrested and cautioned, then they will take his fingerprints and a DNA sample. Then you will go into another room for a formal, tape-recorded interview.
You or your husband can be the 'appropriate adult', but there are a couple of reasons why you might want to let someone else do it. Firstly, the role of an AA is to make sure the young person understands everything that is being said and done; the AA must stay calm and rational, as well as making sure they understand everything themselves; so if you don't feel confident about that, it might be best to get someone else. Secondly, more harshly and personally, if you let your DS deal with the police interview without you, it may have a more shocking 'deterrent effect' - though of course only you can decide whether that's something you'd want. The police must arrange another AA for your DS (usually a social worker) if you decline.
In the interview, the whys and wherefores probably won't be considered relevant. The provocation might be a mitigating factor adding to the likelihood that he'll receive a caution, but it won't 'get him off'. The difficulties your family has faced won't considered at all.
If your DS continues to admit it, this is a first offence, he gives a clear statement and (crucially) expresses regret, then it is most likely that he will receive a final warning or reprimand - these are youth cautions. This is effectively a serious telling off from the police, and then it's over. There would be no fine (afaik fines are only given for civil offences, and assault is a criminal one).
If the charges turn out to be more serious than expected, something important is in dispute, and/or your son does not want to accept a caution, then he can opt to go to court. This means there is the possibility of him not being convicted at all (which is probably very slim, since he's already admitted it), but if he is convicted, then the punishment will be more serious. Cautions are only available instead of court, not after. He'd probably end up with something like a community service order. He might also have to pay court costs.
Given that your DS has admitted the assault, accepting a caution following arrest is definitely the best option if it's offered.
Hope that helps. And particularly, I hope it's not so different in NI than none of this applies!