I hope you arent being serious!
That is 1 and a half feet. You say your dragon is 2 and a half feet!?!
That means the length of your dragon is a foot longer than the length of your tank, and its highly likely its not even fully grown yet. Use your brains
To put it simply, YES, that is far too small! Sort it out or rehome him, thats just cruel.
Why do people take on the responsibility of a living creature without doing research first? Ask questions BEFORE you buy!