2catsandadog hi, my dear this sounds really awful. for the record not all grandparents do this. I have a birth dd and an adopted ds and my in-laws and my mum (dad died before dd was born) have never undermined me as a parent.
As others have said there are no legal rights for grandparents and even the idea of it is quite abhorrent.
I am not going to comment on what you are doing but i will tell you what I would do in your shoes. (read or ignore as you like, I've not been in your shoes so you may sat my opinion doesn't count).
I would arrange to meet my in-laws alone, either at their house or a neutral spot like the pub or soft play pace. Yes home turf is nice but they might if they do not like what you are saying!
i would arrange for dh to join us later with your son. I might or might not tell them know that hubby and child would not be joining us at the start. Probably not.
I would calmly explain that having taken advice from some adoption experts I had made a few decision. Before you start scratching your head we are the adoption experts! (Most of us have a few years of adoption experience under our belts and I've been reading other peoples good advice here for about half a decade) but if in doubt then read up by some real experts like Sally Donovan and Dan Hughes!
I am not sure if others will agree but my advice is that in order to make sure our children are really firmly attached to us (for you maybe say before he starts school) it is important for main things like feeding to come from mum or dad.
I'd also say the child needed to cut down on sugar. Whether a child wants sugar or not is immaterial. You can say a diabetic sugar addict told you that (me!) 
.
Any talk of legal rights would be quickly shut down with a smiling, 'You do know that is totally not true, and anyway, we want you to see your grandchild and we want our child to see you. However, and you really need to get dh on side when you say this, we feel you are undermining our parenting when you override our decisions.
I might say, I am sure you would have felt the same way if your own parents or in-laws had done things you did not want done (e.g. feeding ice cream and sweet treats etc against parents' wishes).
I'd probably emphasis here that a little bit of treats is fine but a small child has a very small body, the stomach is the size of the child's fist and so one ice cream might be a tummy full.
I'd also make it clear that if they made things difficult for you then you would start to shorten the visits or cut them back.
Personally, i would not be boxed into set days and times unless it suits you!
They may get upset, comfort them, but remember (and remind them) they are adults. your child has had a hard start to life and he needs all the adults in his life to be on the same page with his care.
I would not be allowing these people to take my son out unsupervised unless I felt confident that they would stick by my/and my husband's rules about suitable child care.
Re "I had to agree to them taking him for a period of time because I was getting daily telephone calls and "just dropping by" events. They live so close to us, and don't see that as intrusive. Despite the fact that it is."
You did not have to, you felt you had to but really you can say no. I feel that you do need to step up to this challenge and get things on a suitable situation for the sake of your son and your own peace of mind.
I mean this very nicely.
If i were getting pest calls from relatives like this, I would not answer the phone. i would let the messages pile up then ring them and explain i was busy. I would open the door to the callers and say 'sorry we are busy or expecting company or whatever.' But before I did that I would meet them, as I suggest and explain:
We are delighted you love your grandson but...
We need our rules to be respected
We need our son to be fed properly and treats to be limited for his own benefit
We need our privacy
Re " They are just not satisfied with that. It always has to be more. And at their convenience."
They can be as unsatisfied as they like, you do not need to give in to them and in fact when you are faced with this type of emotional blackmail I would cut back on visits and visiting time.
Really, what if in years to come they want to have him over to stay, take him to London/Manchester/Scotland/Wales, take him on holiday, take him abroad without you? Will you say yes if they just pester you enough.
This is not just an adoption issue, it would be unacceptable for any parents but it is, IMHO, even more unhelpful for your son who has been through so much and to constantly have his parents undermined like this is really potentially damaging for him.
Before putting this plan into action, I would talk to my dh, ask for his ideas, then tell him this is what I thought was a good idea, ask for input from him, incorporate any genuine;y good ideas and reject all useless ones.
Your husband needs to make the transition to being a husband and father FIRST and a son SECOND. Those of us who are parents, by whatever means, all do it, we all move from the family of our birth to the one we create. He must do this and support you, otherwise it will be good cop (him) and bad cop (you). And you are not bad cop! You are good mum.
Good luck. 