I don't know about subtle Happygran, I tend to tell my daughter that I'm having pangs to see her twin babies if I don't see them for three or four days. I just long for their lovely cuddles, so I tell her outright, and she tuts at me and tells me I'd better come over. I used to say the same to my son, when grandson was tiny, and would drive over to their house which was 10 minutes away. Ex-DIL was quite frosty with me, but I always made myself useful and would change nappies or feed the baby, help the older children (from previous relationship) with homework, often turn up with something I'd knitted or bought, like packs of nappies or new babygrows. It worked well enough and I saw lots of my grandson.
Why don't you just speak out and say you can't bear to miss a minute, he is so gorgeous? Grandma cuddling baby whilst they can have a meal together, or pehaps even go out for a couple of hours, can be invaluable. Good luck