Then it's double important that he dress for success. There's a reason they called him. My friend had this same situation, and even though it was just a formality, she dressed up, makeup, heels, etc. She was applying for a position, just like a hundred other people that day, but he rehired her on the spot, because she NEVER LOST HER PROFESSIONAL MANNERS. His words. She didn't have to dress up. She's known him for years, worked for him for years. She went in just like the others. She interviewed like it was the first time(even though the owner didn't personally interview her the first time). She put her considerable experience to work for her, and didn't take for granted that it was a given, even though he asked her to come in for an offer.
She left for what she thought was more money and better chance of promotions. What she got was a +$200 a year pay increase, 50lb weight gain, loads of stress, health problems related to this, and her kids missing her, as the owner of the business was a numbers man. She went back with a slight pay cut, from the last job(she declined the higher paying offer, and gave a DAMN GOOD REASON for it) no stress, lost most of the weight, has more time with her kids, and is getting her health back. It's all to dressing for success.
Bottom line. Dress like it's your first time, interview like it's your first time. Don't treat former employers like well-known friends.
Remember, YOU left it for "greener pastures"...I assure, they will. Show them what you're worth.
Just my 2 cp.