I would suggest that for a Macaw you buy him or build him yourself a shower-perch. They are very inexpensive if you buy one online, and they are actually quite easy to build. They are usually made out of PVC piping and fittings, and they have suctions-cups that attach them to the inside of the shower wall...Macaws in-general really do love to shower, and the PVC shower-perches work extremely well for them...
The best way to ease them into getting used to the shower is by first just putting them inside the bathroom with you while you shower. Start-out by just putting a perch for him in the bathroom and away from the shower, or let him perch on something in the bathroom that is away from the shower, and keep the door shut so he can't leave. Talk to him while you're in the shower, and if you just allow him to do this for a week or two, it gets them used to the sound of the shower, used to the steam, etc. And once you feel like he's very comfortable just being in the bathroom while the shower is running, then the next step is to either perch him on the shower rod, or even better if you have a portable play-stand or T-Stand (again very easy to make out of PVC piping and fittings and some Vet Wrap, and quite handy to have anyway), you simply move the stand right next to the shower door, or if you have a tub/open shower just move it near the back of the shower, in a spot where he can see you in the shower and see the water running...And do this for another week or two, all the time talking to him while you're in the shower...Then once he's comfortable that close to the shower and seeing the shower, that's when you want to put him on the shower-perch. At first you want to attach the shower-perch to the shower wall at the very back of the shower, totally out of the water and behind you...It really does help if you have a PVC T-Stand with colored Vet-Wrap around it and you move him to the same type of shower-perch, with the same colored Vet-Wrap. That way he will associate PVC piping with colored Vet-Wrap around it as being "his territory", and he'll always know that those are the places he's supposed to perch...And then just let him sit on the shower-perch inside of the shower with you, but without getting wet at first...and then just gradually start using your hands to kind of "spritz" him with the shower water, and each day move the shower-perch a little closer to the water-stream...A lot of the time they will actually start leaning forward and putting their heads, feet, etc. in the water, testing it out...Eventually you'll get to the point where he looks forward to taking a shower...I've yet to see a Macaw that doesn't LOVE taking a shower on their shower-perch, especially with their owners...
I have 2 shower-perches on my shower wall (they fold-up flat against the shower-wall when not being used), one of them is up higher and most of it is in the shower stream with a small area that isn't, and the other one is a bit lower and is totally out of the shower stream...They typically start-out on the lower one that is out of the shower stream while I'm taking my shower, and occasionally they will move to the other perch and take their own shower while I'm showering, then when they're done they'll go back to the other perch out of the water and wait for me to be done...Sometimes they sit on the perch that is out of the water and wait for me to be done, then when I get out I leave the shower running and they go to the other one, take their shower while I'm getting dried-off and dressed, and when they're done they go to the other perch, and then I turn the water off and get them all dried up...My Senegal and my Green Cheek absolutely LOVE the shower, they'd take multiple showers a day if i let them...My Cockatiel prefers a bath, so she gets a "sink-bath", and my Quaker hates water...She's weird, she just doesn't like it, although she does still sit on the shower-rod while the rest of us are in the shower, she wants to be there, but she does not at all want to get wet...weirdo...
It's all about trying different things to see what works best for your bird, and most-importantly going slowly and gradually acclimating them to it and making sure they are comfortable...But most Macaws absolutely LOVE to take full-on showers, every friend of mine that has a Macaw gives them a shower every single day, some of them with them and some of them by themselves. And they all have a PVC shower-perch for them and it works great.