First, are you sure he's a he? If so, then he may just not be ready to crow yet. He'll crow when he feels it's necessary. And yes it's normal for them to "practice" crow at first - I remember when Foghorn started to crow around 8 weeks old - he'd open and try to blow out a crow - it sounded like a squeeky gate hinge at first. Now he's a full out loud crower. If he's starting to open his mouth to look like he's crowing - he will probably start making a real crowing sound in the next couple of weeks.
Are there other roosters to compete with in the flock? If so he may not be crowing because he's not the lead rooster. My 1st flock had 4 roosters and of them only 2 actually crowed until the day I put 3 of them in a dog carrier for a friend to take them to his house. That day all 4 crowed - LOL.
Now my remaining 26 week old White Leghorn Foghorn crows when he feels threatened or to warn the flock, but my 14 week old Lavender Orpington Francis hasn't crowed yet. I think he knows he's second in command and better not start crowing or he'll be challenging the older rooster. We have a 14 week old Silkie we think is a rooster and he hasn't crowed, but he has made some interesting noises. They are fun to watch when they first start to crow.
They all need to practice before they get it right. If he's been doing this for months, I'd be concerned that there might be a physical reason he's not making any sound but if he's just starting to crow, don't worry, he'll get it.