Hi, with mixed breeds there's no guarantee if his ears will lop, sometimes they lop around 3 months or even later, 5 month. Mixes sometimes have one ear up one down or helicopter ears, sometimes they'd drop even later. As said above you will see when he's fully grown.
He looks like a lionhead at the moment and I agree that his ears are a bit longer but at the moment I can't see any sign that they are going to lop.
Lionheads are also a compact breed so he won't be much bigger I believe, only if his parents were also mixed it is quite possible. I wouldn't believe much what petstore said about parents because why would they say something what could potentially lower the price. Also they might be never seen parents so they usually just give general description based on observation of the litter.
In both cases, with lop or straight ears I don't believe you will love him less or more because of his ears, personality is much more important in my opinion. I have lionhead and lop/mini lop mixes, as babies they sometimes have more lionhead's in them like mane and by 5 months ears lopped and their mane is gone, sometimes appears again when changing their coat for winter.
Lionheads are very friendly in my experience, I've only met one who was a bit mean others were very very nice.
Judging from your photo his fur might be changing as well, those fussy hair can become more tidy on his back and he can keep his skirt a bit longer. He's a very pretty boy, please post some updates sometimes would be nice to see him growing and changing