Hello and welcome. Thank you so much for taking in Speedy and giving him a loving home.

Neko had SRT twice at Colorado State University. In some cases, the tumour can grow back after radiation, which it did with Neko, 3 years after the initial treatment.
This post listed a bunch of places that did treatment as of a couple years ago. Red Bank (New Jersey) does SRT and Yonkers does Cyberknife. Paul recently had Bronx treated with SRT at RedBank. North Carolina State U also has good expertise in treating acromegaly with SRT. Another treatment in the US is hypophysectomy. It is surgical removal of the pituitary gland. A couple years ago it was again around $10K cost, but is only done at Washington State University, that I know of, in the US. The gold standard place for treatment is the Royal Veterinary College (RVC) in London. Similar cost, maybe cheaper flight for you.
Medical treatments are cabergoline, but it's a new trial drug and results are mixed as described above. There is another drug, paseriotide, in two forms, Signifor and Signifor LAR (long acting). One is daily injection and the other monthly. Much better success has been achieved with these drugs, especially the LAR form. The catch? It's around $2,000 a month in the US. Cheaper in Canada, I priced it at $5,000 for the daily version for 3 months supply.

And that's the daily version - my vet couldn't find a source for the long lasting one a couple years ago, when it had just been approved for use in Canada. Last I heard, it hadn't even been approved for use in the UK for humans, much less cats. Hence the ongoing research, again at RVC, for a cheaper, more easily available drug.
In the mean time, to help fight the IAA, and help him feel better, your goal should be to give him as much insulin as he needs to stay mostly under renal threshold. Some people have found that the switch from Lantus to Levemir helps break the antibodies hold. In any case, the IAA is supposed to be self limiting after about a year. But the antibodies don't have a calendar - for Neko it took 9 months and cooincided with her first SRT treatment. Higher doses of Lantus can sting some cats, another reason for a switch to Lev. But we've also had kitties with higher doses of Lantus that didn't seem to have an issue with stinging.
If you are into reading,
this post has a lot of info.
Radiation therapy neuters the benign tumor cells in the pituitary, causing the tumour to reduce, and hence a reduction in the amount of growth hormone going out. It's the growth hormone that triggers the chain of events that leads to insulin resistance. There is also IGF-1 (insulin like growth factor) that is output - it is more responsible for soft tissue growth. After SRT, Neko's insulin dose dropped significantly, but she was still insulin dependant. Some smaller percentage of cats after SRT go OTJ (off the juice). More so with hypophysectomy. But SRT may not change the IGF-1 output. Neko got soft tissue growth in her mouth, and also increasing arthritis. In spite of all that, she lived 4.5 years after her acro/IAA diagnosis and what was probably at least 5 years after she got acromegaly. And most of it, until the end, was pretty good quality of life.
Whew - taking a breath and stepping away from the keyboard for a moment. Feel free to fire questions my way.