HIGHLY unlikely she'd have called the police without a damn good reason!
You say "business" which suggests you have no idea what's happened but HE does. But he's telling you he doesn't. It's not cheap or easy or convenient to move, change number etc. Sounds like he was having contact until that point.
That strongly suggests to me he's fucked up in some way! I dread to think what!
Do you have DC? If so I would not be letting him anywhere near them until you know the full story DEFINITELY not leaving them alone with him.
"All I know is he went to collect the little boy one day and exes new partner and mine had a wee scuffle" correction - all you know is that this is what HE told you! I suspect at the very least it was more than a 'wee scuffle'!
"It was her partner who started on mine" so he says! The physical appearance of her partner is irrelevant! The fact yours already has a metal plate from a previous fight - which would have been with someone else suggests it's YOUR partner starts/finds trouble!
"Police don't normally get involved for a Wee scuffle." Yep especially when it sounds like there was more than one incident too.
WAKE UP AND SMELL THE COFFEE!
19 months - you really won't know him very well. The fact you googled his name shows you don't entirely believe him either. If it's the child he hurt that will be protected and not public record so not googleable.
Your post at 1434 SCREAMS that it's your partner that's the problem.
Your "witnessing" a lack of issue at handovers means nothing, 'wife batterers' tend not to do much in front of witnesses who are likely to be believed.
What 'ongoing family problems'? It gets worse by the minute!
She's even removed the child from a nursery he was presumably settled and happy in AND closed her bank account! Jesus! For her to not even want cm from him - seriously sounding more and more dodgy!
You DON'T need to 'find out more' or do X y z to investigate or help him gain access to the child. Honestly once it's at the point you're seriously needing to consider using Claire's
You DUMP AND RUN!
"What an over reaction." Err no it really isn't! You're describing to us a man who was so badly beaten in one fight he needed a metal plate, who's physically fought with his ex's new partner, who's ex has now flown without warning from her home, closed her bank account, withdrawn her son from his nursery (NO parent does this lightly!! I can ABSOLUTELY assure you - endless threads on here parents debating whether to take great jobs etc because it means taking their child out of their childcare where they're settled and happy), so doesn't even want cm from him, has blocked him from communicating with her, who has called the police on him (on likely more than one occasion), who feels scared enough to call for help when approached by him in public and ALSO confident enough that that if a member of the public HAD called the police at her request she wouldn't be in trouble for wasting their time! Who's older family member ALSO feels scared/confident enough in acting the same when approached by him, who is clearly lying to you about what's really gone on, who isn't doing all he can to re-establish contact with his child, who cannot find a job despite seemingly great efforts to (is something showing up on a background check meaning many jobs are closed to him?)
So NO you're UNDER reacting!
Frankly what she's done sounds to me very like what a neighbour of mine who was a victim of DV was advised to do to protect herself from her ex.
I can see women's aid give very similar advice generally.
www.womensaid.org.uk/the-survivors-handbook/making-a-safety-plan/#1447928860934-dc869210-5f8b
If you live in a small town (which it sounds like you do) then I can understand EVEN MORE her changing nursery, address, bank etc - I live in a small town in Scotland, there's only one branch of the banks, few nurseries etc and EVERYONE knows everyone - I've only lived her a few years but the locals I have on FB (inc relatives) have the most astonishing connections to people I know through a completely different route. We have a women's refuge, most of the women that go there move right out of the area once they've left their ex. The neighbour I referenced her daughter was friends with mine. The kids lost contact in order to protect them (I bumped into her by pure chance in another town over 100 miles away where they're now living. She panicked a little when she saw us which I understand. This was when I found out all that had gone on! I of course assured her that neither I nor dd would mention to ANYONE that we'd even seen them. Dd was luckily old enough to understand well enough).
People can opt not to be on the public electoral roll - it makes things very hard for them in terms of getting credit etc but it would also be something that the police/ss or women's aid could potentially advise as a protective measure.
STOP HELPING HIM FIND HER you don't know what's happened and could be helping an abuser.
"Even though he’d never be able to trace her via the bank she still won’t accept it." Not necessarily true actually but I don't want to say more as don't want op passing this info to her partner. Or other abusers using it - that's why she'll have been advised to change bank account.
I think asking this man 'more probing questions' could put the op at risk - DON'T do that op