When I used a dating site it was very unusual for a woman to show she was interested fast - and with two exceptions (one quickly disappeared and the other... is the reason I'm not OLD) they were scams. So yes, maybe he did doubt it. The "I don't have an app let's use the phone" is also the classic excuse.
Things that alert us men are a woman (well... a woman's profile) moving fast, insisting we're a perfect match like some sort of love bombing, but also wanting to exchange but not to meet. Usually after a couple of days you'll discover that she's stuck in Thailand or somewhere without a passport or money and needs the price of a ticket home in bitcoin😂
Things you can do... Say it straight, but also show you're real by saying you'll be face to face. If you have other tangible signs you are genuine, like a social media profile, you could share that. But you don't know if that's the issue, so...
It could be he's not genuine - a scammer won't disappear, but lying on the profile, so you match something he's not, or he's in a relationship, and realising now that it's stupid? It could be something personal and simple, his old dad needs him... It could be he's not as sure as you are and your speed makes him uneasy... His phone could be out of charge...
Only way to know is ask. But leave room to let him say, "sorry, matching you was a mistake" - or else he won't say it, you'll be stewing in your own juices.
Last point... you are getting attached to a profile. Arm yourself against getting attached too quick to people (not even people: profiles) you don't know well. We've all done it - well ok, mums on mumsnet will never admit to this. I'm a dad on mumsnet, I'll tell you: I've done it 😅- it's not fun. Remember that "no" means "next", and until you have an idea who someone is, not to let yourself too open to heartache.