Undecided Question

What would you do in my situation?

hi , i am a grand mother of 2 beautiful grandchildren. my eldest dominic 5, and my youngest archie 3.
dominic lives with my sons partner (my son is a soldier so is away from home alot.) and Archie lives with me and his mum.and my daughter. (my son archies father died when archie was 16 weeks old he was stabbed to death in a robbery.) archie is very well cared for good mannered boy (i must say i have smacked him a few times but only one firm smack across his bottom each time for being very naughty, yes he cried, no i did not feel guilty because it will benefit him in the long run, teach him to respect his elders and have good manners. mostly he is a very well behaved child, very polite and very well cared for by me ,as his mothers very disabled and in a wheelchair i have brought him up to the best of my ability. he has respect for his mother and myself and even at 3 knows right from wrong, also we go church every sunday, so yes i believe he will grow into a very good member of society. on the other hand i'm very worried about dominic he misses his dad alot, and has to live with his mum, i have seen his mother smack his bottom, and hit him round the head a number of times. he always has bad bruises on him. i have seen cuts on him and very bad injuries that would need urgent medical attention,which he obviously hasn't recieved. a few months ago he fractured his ribs his mother said 'he fall down the stairs' i think that was untrue. he also has burns and bites on him. he seems very depressed and aggressive. he says very negative comments about himself. he has dirty skin,his body smells,he has unwashed,uncomed hair and untreated lice. clothing is very dirty and very small. he never wears a coat in the winter. but last month i brought him a warm coat costing over ￡65 myself, but it is now dirty, but if it wasn't for me he wouldn't have a coat. he is constantly hungry (when he comes round mine like once a fortnight i cook him a nice meal.) he is very skinny and you can see his bones. he is always very unwell but his mother will not take him to the doctors. i cannot afford to keep buying dominic clothes as archies mother is disabled now i have to look after archie buy the food and archies clothes i do not work as i'm lynda (archies mothers.) carer
between us we recieve ￡300 a week with that money i have to pay the rent ￡100 a week buy archies clothes i usually spend about ￡40 a month on archies clothes. and 5 peoples food ￡100 a week . i also save ￡40 a month to send archie to a centre just to give him some time on his own and play with children his own age. every 6-8 weeks i pay ￡12.50 to get archies hair cut. also spend about ￡100 a month on bills we are not a rich family but we have food in our bellies and a roof over our head and each others love you can't ask for more than that. i think i am a very good grandmother but i can't afford to look after dominic aswell i buy him clothes and on his birthday give him about ￡20 as i do when its archies birthday. i spent ￡100 each on my grandsons at christmas. i can't afford to keep buying them presents so they only get presents at christmas and ￡20 each when its their birthdays. i have bills to pay. also my daughter cannot work she is has autism so i care for her too. out of our money we have to buy wheelchairs for archies mum and things that my daughter will need. like i pay for my daughter and lynda to have there hair cut too.i just need to know what to do about dominic he is my grandson i know it sounds harsh but he isn't my responsibility. archie is because his mums disabled what would you do in this situation i can't afford another mouth to feed but i try my best and being a good mum and a good grandma and our children grow up to be hardworking members of society. please plaese help me!!!!!!! do i report dominic mother to social services? thats been bugging me for a while. i need to do whats right for dominic please please help.
thank you.