Thanks for your question on Healthcare Magic. 
I can understand your concern. 
It is very difficult to assume diagnosis in children because they can not speak in detail about their symptoms. 
He is premature and had 
heart murmur, so we should definitely rule out heart and lung related diseases for his 
chest pain. 
So 
consult pediatrician and get done 
1. Ecg and 2d echo to rule out heart related causes. 
2. Clinical examination of respiratory system and chest 
x ray to rule out lung related causes. 
If all these are normal then no need to worry for major heart or lung related diseases. 
Simple 
musculoskeletal pain can be the cause for his symptoms. 
So give him painkiller syrup and apply warm water pad on affected areas. 
Don't worry, he will be alright but first rule out heart and lung diseases. 
Hope I have solved your query. I will be happy to help you further.Wishing good health to your son. Thanks.