No response for Visitor Visa over 7 months (applied thrice)

I am British, lived here all my life, grew up in the South West etc. My partner (of 10 months but have known him for longer) is Serbian. I've flown to Serbia twice since we started seeing each other but past that my holiday leave ran out and awkward flight timings mean I can't do weekend trips.

So we started applying for a visitor visa so he could fly to me instead. We are currently on our third application. Our first, where we did not provide the necessary documentation out of ignorance of the strict regulations was rejected in 4 days.

Our second application was much more indepth and ticked eevry box they asked us for - we gave evidence of him having 100€ for every day he would be in the country, gave my address and demonstrated my ability to support him, plus provided details of his family ties and the degree he is studying so he has ties to Serbia.
3 months passed and we heard nothing back. We emailed them using the email provided and only received back copy paste emails saying that delays were possible. Once we received an email back saying we would get a judgement the next week which never amounted to anything. Eventually I contacted my local MP out of frustration from not actually being able to talk to a real person only to find out that the Home Office had no record of him ever applying.

We took this reply and showed it to the company that handles visas in Belgrade and after a 2 week waiting period declared the application lost. After being issued new documents (passport etc), we reapplied a few weeks later.

It's now approaching the 3 month mark again and we have still heard nothing. Again we received an email saying we would get a reply next week and again received nothing. I emailed my MP again last week and they followed it up for me, only to say that the Home Office still doesn't have it on record - the only record they have is the rejected application. She kindly said that sometimes the system is slow to update and she would come back to me next week, but after nearly 3 months again I am a little skeptical of it ever showing up.
I really don't know how we can progress this. It feels like if we apply again the same situation is going to happen and I don't know what is causing it. There were plenty of Serbs on my flight to and from the UK when I was visiting him, and it's not like he has any dubious family history. And we have no way to really follow up his application other than putting his reference code into the government website where it just permanently says they are considering his application.

Visitor visas are supposed to take 1 month max (with 99% response within 15 days) and we've been fighting the system for about 7 months now. If anyone has any suggestions or advice I would really appreciate it. I could even understand if they rejected him but we just don't have any kind of response.