I don't have any real evidence but my presumption is that it was only 1 Dementor so he didn't need to use a full patronus. The only reason for that logic is when Harry was

trying to protect Sirrius (I'm sure I spelt his name wrong) he was able to use a "whispy shield" of a patronus to deflect a couple of dementors

I don't have any real evidence but my presumption is that it was only 1 Dementor so he didn't need to use a full patronus. The only reason for that logic is when Harry was

trying to protect Sirius he was able to use a "whispy shield" of a patronus to deflect a couple of Dementors.