There are absolutely no repercussions to being a Wereraven
Well, that's obviously your problem in a nutshell.
I'm thinking to balance this you can only be born a Wereraven. The group in Curse of Strahd can bestow one raven cloak that can cause the curse while attuned.
Okay, what you are describing here is a limitation, not a balance. I'm not trying to be pedantic; these are different things and will have a major bearing on your end goal.
What other measures can I take to make it a curse?
Assuming "it's a curse" is your end goal, no amount of limitation will do the job. If being a wereraven is "awesome," then the one player who gets to be born with this will just see it as being born awesome. You may limit him from spreading "awesome" to whoever he wishes, but it's still a boon, not a burden. Sure you have hero lycanthropes like Ivan Dragonov (infected) and Carnagan Wolfe (born), but in both cases their powers are so awesome that they needed additional circumstances to make they tragic. Cursed lycanthropes, sure, but lycanthropy is not the actual curse in either case.
Saying it's a "curse" means that, at some level, this player occasionally wishes his character was not a wereraven, and/or that most other players don't envy his character, would reject his offer if they knew all the conditions, and would get upset if he imposed it on them. Some of this can be imposed by limitations--the part about their clothes not changing with them, for example--but limitations usually just reduce the utility of the powers. You just reduce his "awesome" down to "sweet," or even down to "nifty," but a smaller boon is still a boon, and all the others will still want him to share it.
A curse is a burden. No one wants a burden.
VRGTW suggests several possibilities. As Zilfer pointed out, there's the daily requirement in x pounds of raw meat, which his raven side would probably lean toward grubs and worms. Then there's outcast rating--the social repurcussions if anyone found out he was an unnatural creature (or just noticed him scarfing down a maggot when he thought no one was looking).
And there need to be consequences to stuff like the clothes. So he can't take them with him, he has to undress before changing. That's a mere logistical limitation--a little planning and he still gets to do lots of cool stuff. But what if an emergency situation convinces him to breaks this rule, and he changes while wearing them? Do they get destroyed, or just left behind? Does he take damage? I would recommend that either he or the clothes take damage, at minimum. IIRC VRGtW recommends both.
But even the threat of destroying his favorite magic boots is only a logistical limitation if he has total control over when he changes. That's why VRGtW recommends that he lose control of his powers if he doesn't eat his daily requirement in raw meat. NOW this is a burden, with some teeth. He MUST consume a couple handfuls of bugs per day, OR he runs the risk of changing form spontaneously (i.e. while wearing clothes), with all the corresponding consequences of damage to him, his clothes and his social status. He may invest in a hat of disguise so he can walk around naked and always appear clothed, but this is still something the other members of the party are going to think twice about. Every time they watch him rustle up his grubs, they will see how much of his life revolves around keeping control of the inner beast.
And about that beast: it sounds like you are going with the idea that the raven side is not evil. That's fine, that's in keeping with plenty of other sources in Ravenloft. BUT, if he skimps on raw meat, or otherwise changes form unwillingly, there's one other side effect I would impose. The monster that takes over may not be EVIL, but it's not HIM, either. It might fly off to go investigate something shiny, or turn over a rotten log to hunt for food. Even if you rule that it fights alongside his allies in combat, he won't remember it afterwards. This should only be true of when he changes form unwillingly, but it should be a real threat that looms over him.
Lycanthropy is about someone else living in your skin, and that someone else doesn't have to be a murderous beast hell-bent on killing those you love in order to cause you grief. Let the others in the party think about that while they are babysitting this PC's raven half until he comes back to himself.