Hi ecwinters, congratulations and welcome
Oh how I wish EASY really were easy... What you describe is actually really normal, it can take a few months actually before they start to be able to get over that transition themselves. Just a couple of questions for you before I try to answer yours - how is he currently getting to sleep? Are you using shh pat and how much? Does he take all naps in the cot or are you able to give him some in a sling/pram/car?
Ok so... A) the most common cause of 45min wakings at this age (I think!) is over-tiredness, even at the A time you're currently getting (around 1h 20, right?). Have you tried getting him to sleep even earlier? Both mine were/are short A babies, so at this age it was really just a (long) feed and nappy change before starting wind down, which was frustrating but they just couldn't cope with much more than that, I think our optimum A time was still only around 45mins at 6 weeks, which is shorter than most but not off the scale. Something to consider, anyway? DS in particular didn't show any tired signs until around 2h, by which time of course he was extremely OT! Your description of your DS as going from happy to hysterical in a moment makes me wonder if he's just quite late with his tired signs, and if you see them, you may have already missed that window? Sounds like you're doing well getting him to sleep initially, but using the same techniques isn't working so well after the 45min wake up? Some mums have used the wake to sleep method (I'll find the link...) but I've always been too chicken! Alternatively, at this age, if you can APOP (using AP on purpose) to extend the nap, this can work really well to help them get the sleep they need whilst they're still a bit young to be forming strong sleep associations. Plus of course helping get your EASY back on track!
B) You don't have to have a 3h EASY at this age, you can do 2.5h EASY or even less - do you have a feel for how often he wants to eat? With short A times and short naps, some people find EASASE works, so get him up after his 45min nap, give him a really short, low key time up, then go for another 45min nap before the next feed. You can of course work on extending the nap and give up when he seems hungry or when you think there won't be enough time for another feed before he gets tired again. Or APOP - really before 3-4mo I wouldn't worry about using that as much as needed for your sanity and his sleep needs. If he can have a good 'cheat' nap then he'll be better rested and more able to settle for the following nap. Also I really wouldn't worry about a 12h night, if you can keep track of roughly how much sleep he seems to need over 24h (probably 14-16h+ at his age) then you can use the amount of A time he's had in a day to work out whether the next sleep period should be a nap or night, iyswim? He'll probably need 4-5 naps, or maybe even 6 if they're all short. The night will vary, and often tends to be shorter when they're very young.
C) personally at this age, I'd APOP if possible! As above. From 3-4mo you will want to be getting more consistent with sleep training and this is around the time they develop their own self soothing skills, so any effort you spend on teaching sleep now will help then, but try not to stress over it, I spent the first few months of DS's life in a darkened room and regretted it, whereas I did much more APOP with DD and by 5mo there was really no difference in their self-settling skills
HTH, hugs for this exhausting time, it won't be like this forever