It's important that your brother wants to become independent by himself, and understands why they stand to benefit from it. (For me, living independently means I can go to sleep early if I want, or go to sleep late if I want, or sleep in an extra hour, so long as I'm not late for work the next day.) You don't want to try to force the Echo onto your brother to make him independent if he's not willing -- it will just end with him ignoring the Echo, or worse, trying to break the thing. (I realize that it may be hard to have this conversation with him. It's not totally clear how "high-functioning" your brother is from your post.)
I agree that alarms are a mixed bag. They can cause sensory overload for some; for others they're easily missed/ignored (especially if they go off too often).
An aside: it sounds like you're trying to justify the purchase of an Echo by using your brother as an excuse. Don't do that. If you just want an Echo, you should own up to it and buy it because you want it. But if you already have one, it's fine to experiment with it.
A voice recording alarm clock is a lot cheaper than an Echo; have you tried that? There are also vibrating alarms (the Pebble had a great one, but they went out of business -- the Fitbit "Silent Alarms" are too weak in comparison).