This is a tough one. Up until this point he has established a set of behaviors and you probably have been allowing it. Gradually and nicely put him down when he crawls on your lap. Expect a few tears but know that he/she will adapt slowly. Don't get angry. Just be persistant. Funny thing is when they turn 8-10 they don't want the cuddles and you will.

Maybe arrange for him/her to spend more time with the grandparents? It will give you a break and will strengthen that really valuable grandparent/grandchild relationship. My girls spent loads of time with their nan and grandad as kids and it's wonderful to see how much love there is between them.