Any major change can throw a child off-stride for awhile. And trained kids can have their ups and downs even without a particular disturbance. According to an article on Toilet Learning (http://betterkidcare.psu.edu/AngelUnits/OneHour/ToiletLea..., "When young children are playing, they can forget the physical signals that they need to use the bathroom. Also, children might be interested in using the toilet one day, and not the next. Many children will insist on wearing diapers and refuse any suggestion of using the toilet. Setbacks like these are normal…."
Things will probably get back on track pretty soon. You might be able to encourage his "remembering" with a sticker chart. I'd give lots of stickers the first few days (perhaps one for every dry hour to get him connected to the chart), then gradually reduce stickers as he achieves more consistent success. There might be a special reward when you're ready to retire the chart.