possibly a set, i dont see them calling preflop raises with 45(perhaps), maybe a10,aj, ak. lol but its funny cause ive played with some of those guys before and against you too. i say a10 and i'd fold.

on second thoughts when the ten hit he could be trying to protect his ace and also buy the pot right there.

edit: and looking at it again i'd raise more on the flop probably about 4 or 5$