His contract at Swindon run out in the summer, and he wasn't offered a new contract up front. Instead he was told to improve his match fitness before a decision on his future will be made.
He has been involved with Swindon in their pre season friendlies as an unpaid player, so I'm not sure if he has just come to Forest Green to improve his fitness to be able to re-sign for Swindon, or if he has given up on ever playing for Swindon again and will instead sign for Forest Green.