Just rang Jan ... He's doing very well. He weighs 666 grams which puts him at about 80 days old: very young to survive without a Mum. (See here for a table of weight / feed ratios).
He has not developed diarrhea which is one of the big dangers for young rescued macropods. According to Jan he's doing better than most joeys she's had in her care, which is wonderful considering how very cold he was to start with. He's much more active and very curious about his surroundings. We'll go visit him later this week and take some more photos.