Maybe they're busy with their own lives? Families do drift apart when you have to take flights to visit. It's expensive, time consuming and stressful. Especially in school holidays!
Once every 1.5 years sounds average. How old is grandson?
It's hard for grandparents to have a close bond long distance. How does your son react to visits?
Do you make them feel welcome? Nice guestroom, clean bedding and towels, home comforts like dressing gowns, slippers and favourite toiletries?
Space for them to leave their stuff between visits, a wardrobe or rail to hang clothes? Ideally guests have their own bathroom too. My family visit more often now we have a guestroom with TV and private ensuite! Much more like a holiday, rather than sofa surfing or sleeping on a campbed and queuing for shower. My parents value privacy and siestas.
Is guestroom warm? My mum was too embarrassed to say she was cold. I only noticed when she started bringing her electric blanket 😳
Do they have a good time with you when they stay? Honestly?
Eg do you expect them to fit into your routines and help with childcare/housework (works for some families, but may leave them feeling unappreciated or in the way if they want to be treated as visitors)?
At Easter, I suggest you plan the weeks as if a special, treasured time with your parents. Focus on them. Fun day trips, pamper them a bit. Cook their favourite meals. They probably miss you and would love some childfree time, so get DH to have toddler at times: take parents out for a nice meal, or fancy brunch, or spa (whatever they like). Plan some trips aimed at their interests as well as kid stuff. It gets boring being dragged around zoos and softplay. Good for child to learn about their interests too. My mum loves cathedrals and old buildings, so we do sightseeing too. She loves telling DC about the architecture and taking photos.