M.P.
Since Similac Advance seemed OK, I'd go back to that and leave him on that for at least 3 weeks. Changing formulas does not allow for enough time for his body to adjust. He's been on 3 formulas in 7 weeks. That's an average of a different formula every 2 weeks. His system may not even know what to do anymore. You do not want him to be constipated.
Also keep in mind that some of these symptoms may not be related to the formula. A runny nose can be just a runny nose. A rash can be caused by things other than formula such as laundry detergent, fabrics perfumes, lotions, etc. And, many babies just have a rash from time to time. And spitting up can be caused by having his tummy too full or needing to burp, or lying down too son after eating.
I strongly urge you to go back to the formula that seemed to work and stay with it for at least 3 weeks. If he spits up, try different ways of holding him while feeding him, being sure to burp him more than once, and keeping him more upright for 15 or so minutes after feeding him.
Prosobee is a soy based formula. Do you have reason to believe that he's allergic to milk protein? If so have his pediatrician test for that allergy. I would not put him on a soy formula if he doesn't need it for a number of reasons. First of which is that babies are just as often allergic to soy as they are to milk. And soy contains hormones that may be not so good for your baby.
