You could ignore it. After all, you know exactly what it means - it means the same as "We were swimming". If clarity is all that matters, there's no problem. In any case, Jimmy is writing as he would speak. If we was swimming is good enough when he speaks to his family and friends, who are you to query it?
Or you could cover it in red ink, with a stern reminder of the need for good grammar. After all, we all know that we were is standard, and we are all in favour of maintaining standards. If Jimmy writes "2 + 2 = 5", we mark it wrong. Why not do the same here?
Language of course is different and, unlike arithmetic, grammar is relative. What's right for French is wrong for English; and what's right for standard English is wrong for non-standard. For people like Jimmy - that is, for most of us - we were is wrong, and we was is right. Yes, really - standard English is a minority dialect which carries such clout because of the high-status places it is used, such as education, the law, the media, dictionaries and TES columns.
Can't we at least agree that verbs ought to change with their subject, so standard English is better? In standard, was does change to were when we change he to they, whereas in non standard it stays as was. No, in fact by this argument standard English is worse, because the verb to be is a glaring exception, a messy left-over from an earlier stage: it's the only verb that changes with the subject in the past tense. Every other past tense is the same for all subjects: I walked, you walked, she walked, we walked ...
Non-standard English sensibly treats to be like all the other verbs, with just one past-tense form (was or were, depending on region).
So where does this leave Jimmy's we was swimming? By all means show him how sensible his home grammar is, but he still has to be able to use standard grammar. The fact is that "formal" writing - that is, everything he writes for you - needs standard English. By this he will be judged, and we have a duty to teach him this.
We was hoping you'd like to know all this.
Richard Hudson is professor of linguistics at University College London and has advised the National Literacy Strategy team on grammar issues.
Geoff Barton is headteacher of King Edward VI School, Bury St Edmunds, Suffolk