When I switched to formula feeding after my daughter just wouldn't breast feed any more, she was consistently constipated. I read somewhere that the added iron can do that to a baby.
For your son's immediate need, I would go to the pharmacy and get children's glycerin suppositories. They are very safe. I've used them frequently for my daughter and sometimes I still do if she needs a little help.
They also have glycerine liquid in a tube which works too. But since he's so blocked up now, you need to be very careful that he has the space for insertion. I'd use the stick kind for now - maybe a half piece at first. If you feel any resistance stop. Then try again 5 to 10 min later. If you can get a whole one in, that would be best. If he's really blocked up it could take up to 2 hours to work and it might hurt coming out. If there's just a little blood, it'll be OK. 1 week really is a long time.
I normally did not let my daughter go more than 2 to 3 days without a bm. I could tell it was time for another suppository if her stomach would harden up and she got cranky like her tummy hurt. Also she would not eat as much if she was all stopped up.
I also started giving my daughter 2 oz. of prune juice daily at 6 mo. then increased it to 4 oz. at 10 mo.
This really helped a lot.
Now that she's 2 1/2, I make sure she gets lots of fiber and she tends to eat lots of fruit. Sometimes she eats so much fruit that she tends to have the opposite problem.
Good luck.