Hello and welcome to the forum. There are a number of causes of loose stools. If this is a persistent problem it makes sense to rule out infection first. Campylobacter and Giardia can cause diarrhoea, as can parasitic infection. You would need to talk to the vet about this and to take a stool sample for testing.
You say that your dog is nervous and as it is possible for stress to cause problems like this it could be useful to address this. If you can consult a qualified dog behaviourist for a one to one session it might pay dividends in the long run. Stressed dog are unhappy and prone to health issues. He is young and might respond well to treatment and training. I suspect that we pet owners underestimate stress problems in dogs.
Regarding diet, the Skinners food that you are feeding has the following ingredients:Brown rice 40%; duck meat meal 20%; naked oats; peas; whole linseed; sunflower oil; beet pulp; vitamins and minerals.
The carbohydrate is very high and the protein on the low side of average. That is quite low, particularly for a young dog like yours. This product attracts a nutritional rating of 66%.
We have a thread here
which gives a sketch of what to look for when choosing a suitable food for dogs. As you see, it is advisable to choose a food that has the (named) protein source at the top of the list. If the diarrhoea is diet related the cause could be the high amount of brown rice coupled with oats so if you decide to change food consider grain free. Some have pea and pea derivates and as this is an ingredient in your current food I would be inclined to choose something that doesn't contain this.
Some dogs have allergy to storage mites. These are found in dry dog food, mainly in those containing grains. It is not possible to say if this is the case with your dog but is given raw, wet or home cooked and it improves it might be a pointer to this problem.
An elimination diet is worth consideration but this needs some thinking about and planning. It can take a while to complete but in the end you would know what your dog can and can't tolerate.
We have a thread here
which contains a link to a Facebook video about inflammatory bowel disease. It can take a while for gut to repair itself so it is necessary to be patient.
Regarding probiotics it is good that you are giving this. It needs to be canine specific and a suitable one is Lintbells YuDigest.
If you plan to change food you can source something by using the Dog Food Directory and selecting the appropriate filters. I have done a quick one for you and it returned three pages. If you need help with this, please ask. It is best not to swap and change and to be patient. The nutritionist in the aforementioned video recommends not using dry food for dogs with inflammatory bowel disease but this has budget implications, especially with a large dog such as a Labrador.
Have a think about things and please come back if you need to discuss further.