Here’s a good guide. https://www.instructables.com/Stuffed-Animal-Restoration/

However a seam ripper might be safer than an exacto knife if you have one. Pick a relatively straight/flat seam to open just to make it easy on yourself when closing him back up. I’d also recommend not wringing his skin out but rolling it in a towel and pressing that under a book to remove water, and then air drying. (Wringing can stretch out fabric and dryers can cause shrinkage or permanent shiny spots where the faux fur got flat ironed against the hot drum.)

If you feel a bit out of your depth or don’t want to take your chances you can take him to the Doll Hospital https://thedollhospital.com.au/