He does look awfully thin.
Force feeding a frog.... i shouldn't do it.

You have to know why he is this thin, so you can treat this.
The absence of a feeding response must have a reason and does not stand on it's own.
I'm afraid you have little chance getting this one back up to good health, but you can always try ofcourse.

If possible, try to collect some feces and let it examine.
Altough some treatments on a frog like this may be fatal as well.

You may try putting him in a saline/glucose solution, both of 0,9%
My vet suggested this for imports in extreme bad health as a last resort.
It may aid him in strenghtening up.

You can make this by getting 1 L of warm water and put in 9 grams of Natriumchloride (just table salt is fine, as long as it doesn't contain iodine)
and 9 grams of glucose.
Mix it in good, and let it settle to reach room temperature.
Put in you're frog for about 10-15 minutes.

But most important is you get feces, soon as possible.

Next time, please don't buy frogs like this anymore.
It's awfull how some can sell them in a shape like this.
I think it's outrageous that someone made money on this.
If i were you, first thing in the morning, is going back to where you got it and demand money back.
Sounds cruel to this frog, but if someone has to lose money over this, it isn't you.