We had the same issue when our DS was 3 months old---DS is 7 months now. He's a soy drinker too----we were told that apple or prune juice would work really well mixed with formula, however, doc warned us it would cause really bad gas---and when a babe is already constipated---they don't need an additional discomfort like gas. The other option that was given to us was 1-2 ounce of water mixed with 1/2 teaspoon of brown sugar. Sugar helps the intestines draw water into the stool to help the baby pass their stool---helps soften it up and helps move on!

It worked wonders---without gas pains and issues!!!! We gave it to DS 1-2 times a day for about a week and then his constipation let up and he was regular again. Then we took him off the water and brown sugar. I didn't have to worry about my little one getting hooked on the sugar water part---he could honestly have taken it or left it either way.

He doesn't miss it!

It didn't interrupt with his eating schedule either.
Good luck!