I am so sorry about Picasso. 
There are so many things this could be and of course it is hard to tell until you can get Picasso in to see the vet. I wish I was a vet in this case so I could help! I am sure you will get some good advice on this forum to help keep Picasso as comfortable as possible until you can get him to a vet. Keep him warm, make sure he is drinking his water (additional hydration and nutrients could also come from different soft type substances I have seen people discuss on the forum, like Ensure...do you have that product in England? I thought I remembered that you live in England, yes?
If needed, you can also make his housing more comfy for him by rearranging any steep ramps or things that might be hard for him to get up onto and keep his food and water in a very easy and accessible area. Oh I feel useless making this post. I know that there are probably 20 other things I am not thinking about or forgetting about that hopefully someone else on the forum will remember to tell you. I am so very sorry about Picasso. I wish I knew what was wrong. I know there are others on the forum who have given their rats some over the counter medications like liquid baby ibuprofen and things like that but I hesitate to suggest anything specific because I am really not well informed on dosages and such. Hopefully you will get some good advice soon from the rat veterans on this forum. Please keep us posted on how things go with Picasso? All my love to him.
There are so many things this could be and of course it is hard to tell until you can get Picasso in to see the vet. I wish I was a vet in this case so I could help! I am sure you will get some good advice on this forum to help keep Picasso as comfortable as possible until you can get him to a vet. Keep him warm, make sure he is drinking his water (additional hydration and nutrients could also come from different soft type substances I have seen people discuss on the forum, like Ensure...do you have that product in England? I thought I remembered that you live in England, yes?
If needed, you can also make his housing more comfy for him by rearranging any steep ramps or things that might be hard for him to get up onto and keep his food and water in a very easy and accessible area. Oh I feel useless making this post. I know that there are probably 20 other things I am not thinking about or forgetting about that hopefully someone else on the forum will remember to tell you. I am so very sorry about Picasso. I wish I knew what was wrong. I know there are others on the forum who have given their rats some over the counter medications like liquid baby ibuprofen and things like that but I hesitate to suggest anything specific because I am really not well informed on dosages and such. Hopefully you will get some good advice soon from the rat veterans on this forum. Please keep us posted on how things go with Picasso? All my love to him.