I think
Cass for a boy is very nice and refreshing, but I prefer it as a nn.
Cassanova is a definite no-no to me though. Besides being misspelled (properly, it's Casanova--but see below too), it's too prone to teasing imo.
Cassian's pretty. I also like Cassander (the male form of
Cassandra)
Cassius, and
Cassidy for a boy.
There's also the
Cas- names, which you could nn
Cass too imo:
Casimir (though I don't like its meaning),
Casper /
Caspar,
Caspian, and
Castor (but then there's castor oil to think of). Some would also include
Casey in this group, but I don't because
Casey's pr. KAY-see, not KAS-ee.
Miranda
Proud adopter of 15 punctuation marks; see my profile for their names.