Welcome to Healthcare magic forum.
Green stools with mucus and froth may occur commonly due to the following reasons,
1.Infection or gastroenteritis
either viral or bacterial.
2.Any medications given to baby or mother.
or allergy either to formula or any protein in maternal diet.
4 Hypermotility of gut, or other digestion and absorption problems.
In order to confirm the cause you may consult a pediatrician
for a thorough evaluation and investigation like stool routine and culture tests.
Even in healthy children repeated diarrhoea can cause malabsorption
and hamper the growth and development.
Hope I have answered your query.
Wishing your son good health forever