The loose stools are most likely due to the increased saliva. Now, whether or not he gets teeth soon is a different story -- some kids show teething signs for months before that first tooth breaks through. If he's had a cold recently (even a tiny runny nose that you barely notice), that can cause runny stools, too.
Honestly, I don't think the loose BMs have anything to do with his readiness for solids. If you were already planning to wait until 6 months, go ahead and wait. In fact, starting solids when he's having other "issues" anyway might not be a good idea b/c you won't notice as easily if he's having an abnormal reaction to the cereal.
No one has mentioned this, but it's not necessary to use cereal as a first food either. Cereal is mostly empty calories -- yes, it has iron, but some studies have shown that our bodies don't absorb that iron as well as naturally-occuring iron in other foods and breastmilk. One great first food is banana b/c it's soft, and it's sweet like breastmilk.
All that being said, if you truly feel like he's ready for solids, he's not really that far from 6 months. I started my oldest on solids at 4 months (b/c that was the recommendation at the time) and my next two somewhere b/w 5-6 months. The great thing about waiting until closer to 6 months is that they don't really need "baby food" -- they can eat a lot more table foods, so it's easier to just share some of what you're eating. You can still use baby foods, but older babies are able to eat foods with a bit more texture so don't require the purees that younger babies do.
I know those runny stools are annoying (we've got some messy diapers this week due to my 17-month-old's runny nose), but they'll pass, and hopefully, you'll be back to only 1 dirty diaper a day soon. :-)
S.,
mom to Megan (5 y/o), Annie (4 y/o), and Jack (17 m/o)
P.S. If you haven't heard of it, www.kellymom.com is a great site for nursing moms - tons of breastfeeding info!