Trying, trying, trying...

I have a 3mo. old (8/13) born two months early. He is able to poop every few days but will be trying (straining, red face etc...) consistently for a number of hours if not at least a full day before he is able to actually poop. At Drs. suggestion, I've tried the juices, prune & apple but he is not able to tolerate them. Dr. also recommended Carnation Good Start instead of Enfamil but he is unable to burp w/the new formula. Does anyone have any suggestions or have you had a similar situation?

Thanks in advance...