talk to me about lactose issues

DS has been on soy milk since around 13 months when we thought he was allergic to dairy.  He's not, but he really preferred the soy milk so we just stuck with it.  He eats regular cheese, yogurt, ice cream without any issues that I've noticed.  We decided to try to transition him to cow's milk just so he would develop a taste for it especially for when we are out and don't always have access to soy milk.  I've been mixing half soy and half cow's just to get him used to the taste and he seems to be doing well.  However, this week he's been increasingly fussy and clingy and I've noticed very gassy in the evening.  He goes to bed fine at normal time, 8pm, but has been getting up 1-1.5 hours earlier than normal.  I attributed his fussiness to fatigue, since his naps have been shorter as well.  But now I wonder if the cow's milk could be playing a roll???  The gassines is what made me wonder.  Is it possible to have a lactose issue with milk but do ok with other dairy products?  He could also be teething his last 2 two-year molars, but he wakes up in a good mood and has been sleeping soundly throughout the night.  The whining was out of control today....so we tried some motrin just to see.  Any other ideas???