Personally I wouldn't give soy to an under 12 month old after the discussions I've had with dieticians I know, but I guess you need to do your own research and decide for yourself. Formula can block babies up from the added iron and supplements so it's not necessary an intolerance causing the problem. However if it is an intolerance, there is a good chance that soy will have the same effect - most babies who are intolerant to cows milk protein have an issue with soy also. I don't know your situation but if going back to breastfeeding is an option, this is especially helpful for babies with food intolerances.

If you still have any breastmilk, that is a great laxative, so offering a breastfeed or expressing some and giving it to him could help, even if going back to breastfeeding is not an option for you. Otherwise, trying a different formula is a good idea, preferably one that is not a "gold" as these seem to be more likely to cause constipation. Offering some cool boiled water between feeds might help things move along also.

GL hun, I hope you find something that works