Pinkies seem too little by the looks of him. Get yourself a digital food scale and get his weight to determine appropriate food for him.

10-15% of his body weight every 5-7 days up to 500g is what most people will say...

The problem is that Petco has pretty limited frozen food options. You might have to research to find a rodent supplier near you.
A lot of suppliers have pretty steep shipping costs to get frozen rodents to you.