It is one of the strangest sentences in the Gospels and one of the most useful. I do believe. Help my unbelief.
He contradicts himself inside a single breath and does not attempt to resolve it. Both things are true at once, and he says them both out loud to Jesus rather than tidying the sentence first.
What happens next is the part worth noticing. Jesus does not correct him. There is no lecture about doubt, no requirement that he come back once the contradiction is sorted out. The healing goes ahead.
Most of us assume we need to arrive with a clean position. We wait to pray until we can pray sincerely, which usually means waiting until the doubt has quietened down. It rarely does.
Bring the whole sentence. The belief and the unbelief together, in whatever proportions they are in today. That is what this father did, and it was enough.