Lactose tolerence

18mths old have have been taking S26 with no issue, till recently he keep having dierreahea. So doc suggest taking soy milk for this period and it stop. But end up not pooing for a few days. We tried giving him goat milk, no issue too, this time he poo once every few day. So we thought switching him back to s26 since everything seems back nor. However his dierreahea came back.Any thoughts on this, mummies?#adviceplease