Inserting into custom SQL types with prepared statements in java

Have you tried to cast column to enum?

// Setup stuff up here.
sql = "INSERT INTO foo (suit) VALUES (?::card_suit)";
st.setString(1, 'spades');
st.executeUpdate(sql);

Explained in Convert between Java enums and PostgreSQL enums article of 'A web coding blog' with samples:

INSERT INTO pet (pet_id, pet_type, name) 
         VALUES (?, CAST(? AS animal_type), ?);

--or

INSERT INTO pet (pet_id, pet_type, name) 
         VALUES (?, ?::animal_type, ?);