C.W.
Hi R.,
Your son might be highly intelligent and needing both physical and mental challenges. You don't say if he is in preschool or preK but if not that would be a good option. Otherwise regular play dates, outside preferably, or with some challenging activities. When my kids were little we'd meet other families at the park then have the kids do these elaborate obstacle course races through and around the playground equipment.
The whining and complaining just has to be faced head on with consistancy. Explain that you can't hear/won't listen to whiny voices or that it hurts your ears so they close up! Show him by example what kind of voice you are looking for, including "please" and "thank you". Then (sometimes the hard part) is actually listening and responding to him when he uses the right voice. If he is asking for something that you aren't going to give him, like a cookie just before dinner, you can still say yes but with the "yes, later" technique. This is from author Barbara Coloroso. Kid asks for cookie at a bad time. Instead of automatic "no", you say "yes, later". If child pushes for when, you can give him a specific time like "after dinner" or say "I'm not sure, but later". You might then offer him a choice of water or milk instead, or tell him you need his "big boy" help and ask which job he'd like to do, A or B.
I'm sure you are trying hard to give each child attention, but with a 14 month old leg hugger, be aware that you might be putting him off more than you think. Give him permission to tell you that he really needs a hug or a cuddle if he does. I did this with my daughter when my son was born (same age differences as yours) and she really only used it a handful of times, but it really gave her confidence that she would still be heard.
Hope some of this helps.