As you said, a Tourist Visa (normally called a Visit Visa).
Do NOT ask for 6 months as it will probably be denied, especially if it is his first one.
Ask for a few weeks, or a month. Plans do change so once here he might stay longer BUT this would need to be adequately explained if He was to apply for another VV late on.
You will need a very good reason for him to return to Thailand after his visit, to ensure the ECO that He is not just coming to live with his Mum.