I don't think this boy's issue is mostly a result of bad parenting or the fact that the child was raised by grandparents from 1-3. It is bacause that's how he is as a person!!
The boy was back at age three and with frequent visits from his parents before that. I don't believe there will be a permanent missing of parent bonding as long as the parents are loving and calm.
Unfortunately some kids are just born sensitive and difficult. Your son seems to crave attention and love much more than an average child. That tells he is a bit insecure and need you or others to constantly affirm that he is beign cared and loved. I don't think there is a better way than what you are already doing. It sounds like you and your husband are already giving him lots of one to one time. you need to be extra patient with him and always listening to what he has to share. The child might be manipulative and driving you crazy. But his intention is nothing more than getting attention. Sometime he is doing that just to test if you truly love him. Show and tell him you love him more than anything so that he feels safe.
Also, has your son ever told you he is scared of being abandoned? if so, you need to pay extra attention.
I have a 12 year old boy who is also difficult. But with we accept him as who he is. He is making dramatic improvements and turns out to be rather responsible. It is worst when parents are stressed, they pick up the tension so quickly and acting out as way of self protection. I feel your pain, but it is a process of raising difficult child and there is greater rewards raisign such child. Good luck.