Visitation Modification And Child Support Laws In Illinois

It has been 5yrs since our divorce and I would like to make changes both in support and visitation. Children ages are 10 and 13. My X has had an increase in pay each yr and the cost of living continues to increase as well as the needs of the children. However my X is very fond of his money. He claims that he can't make ends meet and does not think he should have to pay more support. His reason is because he has the children 8weeks during the summer and every weekend and 4 hours on Thursday evenings. When we got divorced he worked 11-7 shift but since then took a bid on 3-11 shift. He no longer utilizes his Thursday evening visitation due to his work schedule.(he volunteered to change shifts and knew he wouldn't be available for visitation). The children have expressed that they would like to have every other weekend with me and so would I. Their father states "all you have to do is ask for the weekend, Have I ever denied you before? Usually when a person ask nicely they get what they want.". He does not want to change visitation because if he gives up some of his visitation he will have to pay even more in support. The children want this change and so do I so that we can make plans and purchase tickets knowing that their father is not going to deny us the weekend and thus loosing money used to purchase tickets. Where does one start? What are Illinois laws on visitation and support modifications?