Well, your request made me think of 2 things. First, little tiny babies that are only 3 weeks old often don't have any schedule, so that's totally normal and should get better around 6 or 8 or 10 weeks of age. Second, another vote here for a possible dairy sensitivity.
I also have a baby who is sensitive to dairy - and because I'm breastfeeding him, once I eliminated all traces of cow's milk products from my diet, there was a wonderful, VERY noticeable difference in my little guy's comfort levels - he felt soo much better! Yeah, I really miss yogurt, ice cream, butter, etc., BUT definitely worth the benefits. My little guy now sleeps for hours in a row (w/ dairy, he wouldn't sleep for more than 20 minutes at a time and would only sleep when being held), he was much happier (the sad, painful-sounding crying stopped), and his skin improved (what I thought was baby acne and some dryness or a little eczema disappeared and his beautiful, creamy skin revealed itself).
If you're breastfeeding, here's a great list of dairy terms to watch out for if it would turn out that your son is sensitive to dairy:
http://www.kellymom.com/store/freehandouts/hidden-dairy01...
And this is a good primer to help determine what is fussy VS what is food sensitivity:
http://www.kellymom.com/babyconcerns/food-sensitivity.html
If you're formula feeding, be wary of going from cow's milk based formula to soy formula because around 50% of kids who are dairy sensitive are ALSO soy sensitive. This means possibly having to shell out big $$$ for the hypoallergenic formula (which would be the healthiest thing if you have a baby who is both dairy and soy sensitive). But honestly, breastmilk is your best bet - you control exactly what's in it! No guessing. And at 3 weeks of age, you can absolutely switch back to breastfeeding (teach him how to latch) or at minimum pump and feed him breastmilk. He's most likely to outgrow any sensitivity if you a) remove the allergen and b) breastfeed for at least 12 months (with gradual introduction of solids when he is sitting up well, has a good pincer grasp, etc.). If you would want to get him back on the breast, I'd recommend planning for an intensive "breastfeeding bootcamp" for these next 2 weeks before you have to return to work, and you'd probably want to have the support of a good lactation consultant or LLL leader. The benefits of breastfeeding are so enormous that there are too many to list here, and even though the first 1-2 months of breastfeeding can be difficult, in the long run it's so worth the effort. Keep in mind that a nice, double electric breast pump is far cheaper than the cheapest formula - and why pass up the protective effect that breastfeeding has since it lowers the risk for cancer in BOTH you and your baby? (Important to me since my mom had breast cancer.) Also, we're headed into cold & flu season, too, so at least breastfeeding through that would provide amazing immunological factors in breastmilk that no formula or vaccine can duplicate. Breastfeed even longer (10+ months) and you'll get the superior facial muscle and bone development that breastfeeding helps develop (uses different muscles than eating from a bottle) which results in better teeth alignment, earlier speech, etc.
Anyway... whatever feeding method you ultimately end up using, I hope your child does not have any sensitivities to food, and if he does, I hope you're able to identify them quickly so that you're ALL more comfortable!
Congrats on your new baby!