Coronavirus lockdown is having a big negative impact on many grandparents relationships with grandchildren especially if there is distance involved and if grandparents don't feel the same about technology as means of replacement contact.
While your partner's parents may be in the rare group of GOs who do not wish to be overly involved with grand children's daily lives, I expect your partner's parents may be the type that would like pre arranged visits by you but would not ever make them themselves invited or when it's not allowed. It's difficult if a meet up in the park is not possible for them if they are shielding, or not what they would normally do, in normal times. I suspect they do like to hear your family news but maybe aren't overactive in reciprocal contact . Take your lead from your partner, in terms of managing the relationship, but don't give up. I do not know if the pictures you are posting of your son and his other grantdparents are disproptionate in number to your other photos, which they might feel hurt by. Why don't you post things like your son's drawings on line, or doing other activities, and pictures of him with his dad i.e their son. Include little messages to say the grandson is really looking forward to seeing all his grandparents very soon etc. Did they send a birthday and Christmas presents and cards? I'm sure they don't want to feel estranged from their son and his family, but are simply finding it difficult. We will be out of lockdown eventually.