After you beat Dio you must go to your room and change your clothes and then speak to George (the order in which you do them might be swapped just so you know) after which you can then go outside and from there go to the Speedwagon area (but I advice you to go to the shop in Downtown first
You should be able to beat him with your current level, just repeat the battle a few times. If you have run out of healing items then I advice you to start from an earlier point in the game or even from the begining (I'm sorry for making it that hard :( ). Thanks for the feedback tho.
