What a lovely little foster dog! My advice would be to slow down a little bit. He has had so many things happen to him in the last week or 2 - no wonder he is unsettled and stressed. If he came from the dog pound, you also have no idea of his previous history I suppose.
Things like the dog groomer, taking him out for exciting walks etc, it all really needs to wait. He just needs to adapt to his new environment and then you can slowly introduce new experiences to him rather than flood him with so many new things all at once.
It is very common for dogs coming from the pound and going into foster to have upset tummies. It should settle in a few days. He needs to be on a high quality complete dry food - there are many good ones and many terrible ones. So do your research and read the ingredients labels! I would recommend Burns or James Wellbeloved, and definitely avoid Wagg, Bakers or pretty much any of the dog foods that you can buy directly from the supermarket.
You can train a 'watch me' behaviour in the following way. Have a high value treat, such as a small bit of chicken. Hold it in the air with your arm out straight at shoulder height. Do not say anything - this is key. Watch your dog and wait for him to look at you. As soon as he does, then click (if you using a clicker) or simply praise and give him the treat straight away. Repeat a few times and do this several times a day. The idea is that he learns to look to you regularly. This is really helpful when you are out and about.
With regard to him barking at other dogs, this is not uncommon for little terriers. Try not to let him get into situations where he feels uncomfortable and is bombarded by other dogs - you are his protector. Don't rush the socialisation process and just let him settle into his new life and get to know and trust you first.
I would suggest attending training classes once he has had time to settle into his new environment properly. Just take it steady and it is also not unusual for rescue dogs to display different behaviours the more and more they settle. You just can't judge them in the first few days as it is likely to all change as they get their paws under the table so to speak.