* Is the wizard still invisible while he is unconscious?
Yes. For invisibility the spell ends when the duration expires or "the subject attacks any creature.". Improved invisibility removes this restriction so the spell will only end when the duration expires, or if it is dispelled.
* Will the party's cleric need to spot him in order to heal him?
Yes. The hapless Wizard is invisible, so the cleric will have to spot him, but see the next answer as well.
* And if he spots him, will the heal have a miss chance like the attack?
Yes. (Sort of)
If the wizard is unconscious they will be helpless.
You wouldn't normally roll to attack to cast Cure Light Wounds on a merely unconscious target, it's assumed you can poke them - similar to how you can automatically hit when applying a coup-de-grace The problem is knowing where they are when they are invisible.
A creature can grope about to find an invisible creature. A character
can make a touch attack with his hands or a weapon into two adjacent
5-foot squares using a standard action. If an invisible target is in
the designated area, there is a 50% miss chance on the touch attack.
If successful, the groping character deals no damage but has
successfully pinpointed the invisible creature’s current location. (If
the invisible creature moves, its location, obviously, is once again
So the cleric will have to grope around to find the unconscious Wizard, once they've located them with this method they can then cure them.
The cleric might want to get help from the rest of the party to find them, or maybe that spreading pool of blood on the floor is a helpful clue if the GM is feeling generous.