I'm the yin to Carlos' yang .....
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Let's take a book out of the guy's manual, go for it...He expressed interest before maybe he was getting mixed signals and gave up...Strike up a conversation with him, if you need a starter, ask him to help you find some obscure product, if he tells you the aisle number, tell him you checked there, smile and say something like ..
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"You'd think i'd know it by now I've been coming here so long, could you help me find it?"..As you walk towards the aisle, continue chit chatting bringing references to knowing each other, maybe say you haven't seen him in a while and then give that blushing little smile we do ... and say "cuz I have to admit I kinda look forward to your (fill in the blank - little jokes, friendly smile) it makes grocery shopping so much more enjoyable"...
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Angie..

Yep, ask him out, you're got nothing to lose, or maybe in your conversation the next time say 'oh your girlfriend must like your new hair style' or something. If he say..'yes yeah she does', smile and gracefully take your shopping and walk away. If he says 'no I don't have a girlfriend', then jump right in!..

It is weird that he's now ignoring you. I wonder why. Maybe he started dating another customer, and she's super jealous and asked him to stop talking to everyone in the store...
Or, maybe he figured it was never ever going to happen with you so he decided to give up...
Ask him out. The worst he can say is no...

Most guys are pretty straightforward. WYSIWYG. They aren't as complicated or as nuanced as most women. That different treatment might be several things from bad day at work to having..given up on you ever being interested. ..
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One thing I've learned from dating these last 3 years is that men are just as afraid of being turned down as women are of not being asked. So take a chance, if you want to dance...

It could be that before, when he thought he didn't have a chance, he was just all bubbly and carefree. Maybe he senses now that he might just have that chance. Maybe he's becoming shy because you are now responding to him more than before when you weren't available...