Can someone with medical knowledge help?

My son is 2 1/2 years old and weighs about 23 pounds. At one year he weighed 20 pounds and he was weighing more for a while and then he's stabilized around 23 pounds. He eats a lot and eats at least 3 meals a day and fruit for snacks. Now I'm not worried about his eating habits as he eats really well, but his energy level is very horrible. I have an appointment with his doctor in a couple days because since he started walking at 17 months, he just does not have very much energy throughout the day. Whenever I take him to the park he will play for 10 minutes and then go lay down on the ground. He goes to early intervention and by the end of cl he is falling asleep.The cl is about 1 1/2 hours and honestly it's not that physically demanding. We have had him checked for a heart defect and he had an innocent stills murmur. He sleeps probably between 10-12 hours every night and gets a nap midday because he can't make it through. I honestly thought he would grow out of this, but sadly he has not and it seems to be getting worse. My son does have very low muscle tone. We are thinking he may have a food allergy of some kind that may be causing this or it could be something else. Any suggestions from you guys would really help. I'm going to be asking the doctor for allergy tests, but anything else you guys can think of to have him tested for would really help me out when I go in a few days.