How To Treat Constipation In 8 Month Old Child Which Started After Giving Cerelac?

Hi.
My little boy is now almost 8 months old.
For the last 2 months he has been terribly constipated. He doesn't poo for 4 days, then when he does he screams in absolute agony.

Originally when I was still breastfeeding he would poo 3x a day, but due to latching problems which lead to my milk supply dropping etc, I eventually had him only on Nan1, since he has only been on formula his poo's have always been 'less soft' but they haven't hurt him.

We started solids shortly after he turned 4 months old. He was first on Nestum then on Cerelac.

Since his constipation started I have given him prunes, prune juice, rooibos tea with brown sugar, laxette and a glycerine suppository. Nothing has really helped. I took him completely off cereal for a few days, it did come right, but then it went back again.

My sister-in-law has told me of a Novolac product which helpef her little girls constipation, that is what I intend to try next.
Please help.

What more can I do?
Maybe put him back on Nestum?

Thanks
Natalie