I had 2 massive clots (massive bring a medical term - there's massive, submassive (which your DH's clots sound like - but worth asking the medics), and low risk) which almost saw me off back in 2018 after leg surgery.
Mine affected my heart significantly as I had a saddle PE which meant it was sitting in and down both sides of the junction where the 2 pulmonary arteries branch off, so almost no blood was getting through and was backing up in my heart, causing it to swell up a lot.
I was immediately treated with unfragmented heparin every few hours whilst I was in hospital and they basically hovered over me with a clot busting drug in ICU in case I crashed (they wanted to give it to me in resus by my orthopaedic surgeon said he'd have to amputate my leg if I did as it would bleed uncontrollably and kill me - talk about a rock and a hard place!).
I was eventually discharged after 2 weeks with Rivaroxaban to take daily. Which is similar to Apixaban insofar as there is no INR to monitor as with Warfarin. I did become very anaemic on it but that was a combination of being female (i.e. having periods) and being on a blood thinner, so, due to them knowing it was a provoked PE they were happy for me to come off it 3 months later. If your DHs was unprovoked I would imagine he will be on thinners for life (as many people are for various reasons).
I was so so lucky and the doctors were surprised that I survived it considering the size and location of it - I became a bit of a minor celebrity during my stay and had to speak to endless students when I was well enough!
I think the main thing for your DH is for them to find out why he developed these clots - were they "provoked" (i.e. caused by an event, in my case surgery) or "unprovoked", as this will influence the ongoing treatment. I would imagine they'd test him for things like Factor 5 Leiden (blood clotting condition) and other conditions that increase the chance of clots.
The side effects for me were initially pain in my chest whilst my body continued to break the clots down, immense fatigue and some serious PTSD which I ended up seeing a clinical psychologist for, who treated me with EMDR, which worked well. I will never have the lung capacity I once had due to residual scarring and have been left with some small changes in my ECGs (which I've written down on the health app on my phone to tell any medics in the future so they know it's my 'new normal'), but have been signed off by all the medics after having follow up scans and echocardiograms to check for longer term damage.
There is one complication called chronic thromboembolic pulmonary hypertension or CTEPH for short, which can occur when the body doesn't completely break the clot down, which is serious, but can be treated with surgery (unlike the normal form of pulmonary hypertension).
I'm sending you both loads of hugs and luck - they are nasty little blighters which do leave physical and mental scars, but life post-PE is pretty normal for me (and my god, normality was my dream immediately after the PE), and other than a slightly increased risk of developing another (I now take aspirin before flights or any other situation I may be sitting in one position for a while) have been left with minimal/manageable damage.