Could it just be his initials? Or are dates involved? 'I's and 'J's look the same in some scripts - could it be short for 'Ianuarius? (January) Just a thort.
The standard parish register abbreviation for 'James' seems to be 'Jas'. For 'John' it's 'Jno'. William is invariably 'Wm', etc. 'Christopher' is often 'X-pher'. I haven't seen 'JNS' in my travels - how sure are you of the handwriting is my first thought? Can you show us it within the context? That would be interesting!
I spend a lot of time with 17thC, 18thC and 19thC parish records and 'Jonas' is not a name I've ever
come across, for the 18thC. Not that it didn't exist but it's so uncommon that in the random sample of the few dozen Anglican records we trawl for our research - we haven't seen it. Maybe it would be more likely for a Non conformist for the 19thC, but 18thC it's less common - not impossible but... you'd be amazed at their conformity...
Until the mid 19thC you can find entire families on the IGI
if you have a surname, randomly feeding in: William, John, George, James, Henry for the males and Mary, Ann, Mary Ann, Hannah, Jane, Elizabeth for the females. Not to say it will catch them all but it will start you off!
Surnames as forenames is a big Yorkshire phenomena - but much less so than you'd imagine and rarer - but of course not unknown - elsewhere - depends on date, region but also social class. I have an 18thC ancestor with the forename 'Wilson' and a 19thC one whose first name was 'Newrack' - both Yorkshire. As abbreviations are only for names so common that anyone could decypher them - it's not likely to be either 'Jonas' or 'Johnson' for that reason. Hope that helps!