No. You can only vassalize nobles of lower rank. When you win the war, he will thank you and leave to rule Scotland, completely independent from your divine supervision. Drat!
The only way to vassalize a king, is to become an emperor. Emperors, being the highest possible rank, can thus never be vassalized.
As a sidenote, this also goes for titles that are achieved later. If your lower tier vassal gains a title that is equal or higher rank to yourself, he will leave your realm and take all his lands with him. For this reason, you must be veeery careful-like, when you give titles to people who are likely to inherit in other realms, or whose heir are ruling somewhere else.
Of course, this being Crusader Kings, you can do some scheming for a brighter future. When you invite him to your court, you can propose a matrilineal marriage to one of your dynasty. Since he is in your court, he will never object. That means, when he takes the throne later, his heir will be of your dynasty – this may require that any current wives and children experience unfortunate circumstances, resulting in their sudden departure from the mortal realm (deus vult!).
Now, if you're really cunning, you can arrange for him to marry your female heir, and thus two generations down the line, your grand heir will inherit the throne of Scotland. Of course, this won't work if you have any sons ... unless accidents happen or they are otherwise ruled out of contention.