I did read that you have tried changing once already, but, unfortunately, like the first thing we try, sometimes the second isn't quite right either.  Good luck....I know how awful it is to see them go through so much discomfort.  I felt my last comment really was useless for your case, so I wanted to find more info, so this is the reason for adding another comment

I have cut and paste some ideas from http://www.cyh.com/HealthTopics/HealthTopicDetails.aspx?p=114&np=304&id=1805 - please take note of the pro-biotics idea mentioned towards the end.  I have also added some extra info on probiotics from the same site at the end.  I know this works, but you need to research how much to add for your child.

What you can do

• If the baby is bottle fed, check the formula tin to make sure the formula is being made correctly. Measure the water first, then add formula powder.
• Offer extra drinks (water).
• Give brown sugar, one half to one teaspoon mixed in a small amount of water (30 mls) separate from milk at the beginning or end of a feed 3 or 4 times a day until there is a soft poo.
• For babies 5 months and older, offer fruit juice (especially prune juice) 1 part juice mixed with 3 parts water.
• For babies 5 months and older who are already having solids, give your baby stewed prunes, stewed apricots, or steamed vegetables.
• For babies 9 months and older, offer your baby cereals which contain bran. Note: sometimes diets which are high in fibre can cause tummy pain. If your baby seems more unhappy after you start giving him extra fibre, give him less.

Note: Some research shows that using formula with LCPs, or with probiotics, may help babies to have softer bowel movements (poos) than ordinary formulas.

Should LCPs, beta-carotene or probiotics be added to formula?

• Breastfed babies get lots of extras in their milk that are not in formula, and it is not always possible to know which of these are important.
• Breastmilk changes to suit the baby’s needs from day to day in ways that formulas can not – for this reason it is important to breastfeed if you can.
• When you need to use formulas, the closer they can be to breastmilk, the better.
• Breastmilk contains LCPs and beta-carotene, so if they can be safely and readily added to infant formula it is probably worthwhile, particularly for very young babies (the first weeks of life).
• Formulas with LCPs, beta-carotene or probiotics in them may be more expensive, and no formula has all three at the moment.
• It is important that you do not feel guilty if you can’t afford them – there is no strong evidence as yet of any long term benefits.
• Infant development and growth is very complicated. It depends on the interaction of many very different things. All these additions, even if they are helpful, are only some of the things that affect the development of a baby, and many babies in the past have done very well without them.