Amylase is involved in anti-inflammatory reactions such as those caused by the release of histamine and similar substances. The inflammatory response usually occurs in organs which are in contact with the outside world, ., the lungs and skin. These include skin problems such as psoriasis, eczema, hives, insect bites, allergic bee and bug stings, atopic dermatitis, and all types of herpes. Some lung problems including asthma and emphysema may require amylase plus other enzyme formulas depending on the particular condition.
It is great that your horse has 24/7 turnout out and access to pasture. Horses with enough access to good pasture can easily meet essential fatty acid requirements. For your horse to gain weight, you will need to ensure he gets more calories in the diet and to avoid highs and lows you will want to control the amount of starch and sugar per meal. It is important to identify if he is reaching his daily digestible energy (calorie) requirements for his activity level and life stage. If you are having trouble keeping weight on, you might consider checking to see if you are feeding the products you mentioned (alfa a oil and purabeet and conditioning cubes) at the recommended feeding rate for your horses life stage and activity level. You are feeding many products that are higher in fiber and they are forage type products which are good for horses but they tend to be lower in calories. Hay, beet pulp, grass pasture, and alfalfa are all forage type products higher in fiber but lower in calories and have 1/3 less calories compared to grains. Sometimes these can fill the horse up without reaching daily calorie requirements. The Alfa oil with the added fat has higher calories compared to the other products you are feeding and is closer to calorie content of grains. I recommend double checking the recommended feeding rate of Alfa oil and see if you are feeding within the recommended level and increase the amount if needed.