When I'm being taken off of one and put on a new medicine, it's usually over a 7-10 day period (typically - depending on the dosage; I'd drop 25-100mg of the old one and gain 25-100mg of the new one). As far as getting used to the medication, I took several months to adjust to Keppra (it caused me to have mood swings) even a little bit and eventually had Lamictal added in which seemed to be a decent combo.

I don't know how severe his case is as far as observation goes, but if he's a fairly mature fella; just have him call his doctor (or you ) to report any changes in activity. Make sure you both know all side effects and keep an eye out for them as well as a rash.

Hope the new meds help out!