Oh dear, baby still constipated,what next?

Hello again Dr Mom, so my little 5 mth old is still constipated! I got some kid-e-reg and have been giving him some of that on a teaspoon every day. I also drink prune juice and give him diluted prune juice. The only time he will have a poo is with an infant suppository and then it's very thick and smelly. He can go many days with no bowel movement. I don't know what to try next. He has mainly breast milk but one bottle of formula a day as I am back at work full time and just can't produce enough milk to satisfy him even though I'm pumping twice a day and bringing that home. This all started when I tried him on rice cereal, which he hasn't had in over a month but it's really messed him up. He's not on any solids yet. Im afraid to even start those seeing as he won't poo regularly yet. Should I try something else other than formula? Should I try probiotics. I'm worried because I've had a problem with constipation all my life and worry that I'm passing this problem to him. Thanks again for all your advice so far. Samantha Tweet