Hello and welcome to the forum. How awful it must be for your little dog to have this problem. It is horrible seeing a dog in pain. Obviously it is difficult to say what will work but I can see your point in looking for something low protein. One of my dogs has occasional reflux gastritis which I appreciate is not the same as IBS but she too was prescribed low protein by my vet so I had a similar problem to you, only not so severe. My vet suggested lower than about 22%. Did your vet give a figure? It is helpful to know what you are aiming for.
I needed grain free so I searched the food directory on this website and located these products:
Fish4Dogs Weight Control
Wafcol Salmon and Potato
My choice was the Fish4Dogs because I have used their products before with no problems. Also, it has spirulina which is supposed to be good for digestive health. The protein is 23% and it is poultry free so may be suitable for your Frenchie. The bags are only 1.5kg so there is no need to spend a lot on a large amount that might not be suitable. Delivery is very reliable and is free but if you pay a bit more they will send it more quickly. There are a few shops that sell it but not that many.
You can get lower protein foods if grain containing foods will be suitable. Here is how to locate them:
Go to the Dog Food Directory
Look for Filters on the left of the page.
Go to Avoid Ingredients and place ticks in the boxes for poultry product.
Go to Nutrient Levels at the bottom and move the protein and fat sliders to the level you prefer.
Click 'Go' and you will see a list of products on the right hand side. You then need to go through each one individually to decide which might be suitable.
Whatever you try, please weigh the daily allowance in the evening, put it into a small container and add warm water to it. Put the lid on and by the time you need it next morning it will be nice and soft. Crumble it up and give your dog four small meals per day, making sure that the last one is given at bedtime and the first as soon as he wakes. In other words, you don't want to let his stomach to get empty.
There is anecdotal evidence that a raw diet helps in conditions like this and I have a friend whose Chihuahua had very similar symptoms to your Frenchie. After trying many different foods, raw completely turned it around and the little dog is now well. However, he is only able to tolerate certain types of raw food. If you are minded to go with this you will need more advice about it. Raw is not classed as low protein though and under the circumstances it would be best to talk to your vet about it. Some companies such as Honey's, Natural Instinct, Natures Menu and Nutriment specialise in complete raw meals and would be able to advise.
Buscopan is an anti spasmodic which may help your dog. Ask your vet if it would be suitable and what dose would be correct. It can be bought fairly cheaply at the supermarket.
Please keep us informed of the progress of your rescue dog and I do hope that you can find something that will suit him.