10 year old, high RBC low MCV and MCH

my son is a football palyer. gets his blood tested on a regular basis but we have never been asked to run further tests. for some time we have noticed that he gets fatigued more easily than his other friends. he gets short breath (kind like asthma), sweats a lot. His typical blood test results are following:
RBC: 6,17
HGB: 12,1
HCT: 34,6
MCV: 56,1
MCH: 19,6
MCHC: 35
RDW: 18,9
We have made recently new test becasue he looked anemic, had week long diarrhea. Results below:
RBC: 5,65
HGB: 10,9
HCT: 32
MCV: 56,6
MCH: 19,3
MCHC: 34,1
RDW: 16,8
Ferritin: 39
We are waiting for the DNA test but that takes another 2 weeks.
My blood test results  are similar. High RBC, low MCV and MCH so this is for sure gentic thing.
Is it possible to tell now that it is some kind of thal?
If DNA test confirms thalassemia, what other tests are necessery? B12, D levels?
Is diarrhea typical for thal? What is the reason, how to prevent it?
What can I do to help my young football player to increase his stamina?

thank you for your help and answers

High RBC, low MCV and MCH

These are all found in thal minor. Was a hemoglobin electrophoresis test also done?

Athletes with thal minor find they have to pace themselves and to recognize their limits. Tennis legend, Pete Sampras is thal minor and used a strategy of dispatching opponents as quickly as possible in order to not become fatigued. Your son needs to take advanatge of all rest periods he gets, stay well hydrated and follow a good diet and a strong nutritional program that recognizes the depletion of nutrients by thal minor. A full iron panel should also be done, as his RDW is >18, which is borderline for iron deficiency. Thal minor and iron deficiency can co-exist. If it was iron deficiency only, the RBC would be expected to be much lower. With an Hb of 12, he may not have iron deficiency, but the iron panel should be done to check it, and also make sure he isn't given iron if he isn't deficient.

thanks Andy.
Thal here in Poland it is extremely rare condition. So rare that none of my family members  was ever informed to make more detailed blood tests and we all have similar blood test results. Our hematologist is saying that last example of thal she had more than 5 years ago.
We are still waiting for the results of genetic test of my son. Doctor didn't ask to do the hemoglobin electrophoresis or full iron panel which for me is strange because it is available at the local laboratory. When asked what kind of diet or supplementation would be advised she mentioned only folic acid. I'm very afraid that due to rareness of thal in Poland I might struggle to get proper help. Don't know what to do, I don't feel being in position to tell the doctor which tests he/she should recommend to be done. I also don't want to supplement him with all the vitamins which you are talking about on this forum.
Looks like I need to be patient till results come and if thal will be confirmed I will search for hematologist specialized in thalassemia. There must be at least one in Poland. 
My son loves football, it is all his life. If there is any chance to help him improve his stamina we need to try it.