"And ofttimes it hath cast him into the fire, and into the waters, to destroy
him: but if thou canst do any thing, have compassion on us, and help us. Jesus
said unto him, If thou canst believe, all things are possible to him that
believeth. And straightway the father of the child cried out, and said with
tears, Lord, I believe; help thou mine unbelief."
Therefore, there is no reason to think that He would be incapable of casting out this devil. However, the Lord is not always willing to do what He is capable of. Our society has completely turned into the state of Missouri. They think that if you do not "Show me" then you have not the ability to do it. So, our Lord is always capable, but not having the desire to do a thing does not negate His power one iota. Many times, He does not do something for a time for a particular purpose. He waited in the days of Noah while the ark was a preparing. Could He have spoken the ark into existence and immediately flooded the earth? Most certainly! But, His will was for Noah to build it, while He was longsuffering.
He waited 430 years before sending Moses to deliver the children of Israel out of Egyptian bondage. He waited many thousands of years before sending His Son to redeem His people. He is currently waiting now before burning up the elements with a fervent heat. Does He have the power to do it now (I speak this foolishly)? His power is always present, but His will is to wait until the appointed time to do so. So, our Lord straightened out the unbelieving dad, and then cast the devil out of the child. May we ever be found acknowledging Him, not questioning His ability or word, and asking for our will to be aligned with His.