pyloric stenosis at 2weeks old and tummy time

I found out my son had pyloric stenosis at 2weeks old (just slightly under) as he was projectile vomiting constantly and we were in hospital trying to figure out what was wrong with him until he was 1month and had lost a great deal of weight and had to have an IV a feeding tube and to be aspirated every 2hours. After finding out he had P/S we had to wait a further few days for his blood to be corrected then he had his surgery and recovered right away. However in this time he rarely had tummy time and after for a few weeks I didn't do it as I thought it would hurt his stomach or rip the stiches. His 2months 2weeks now and I've slowly started him again with tummy time but he still has a stich in and he screams every time he is on his stomach I'm not sure if this is effecting it or his stomach feels uncomfortable or he his just not used to tummy time yet? But any advice would be great as I've heard no tummy time can be bad for babies ( he can hold his head perfectly when being held up right and can sit up nearly unaided and tries to pull himself into the sitting position)