I've been thinking about your situation and some other things you may wish to consider too.
If your son was to be ill and physically sick then the school and afterschool/childminder/school holiday
club etc have a policy that the child cannot attend for 48 hrs after the last vomit in order to try and keep the bug from being passed on. Therefore, you will have to take time off work to care for him with no notice to your employers at all. You may wish to check with you prospective employers what their policy is for single parents/flexible working etc. My employers are very good and we are allowed 3 parental days per year without losing pay or holiday in order to care for our children in an emergency, but these days are only so that we can find longer term care if it is required. Not all employers offer this, so you may need to consider in the event that your son becomes ill with a bug for a few days, or gets chickenpox etc how you will cover his day to day care over a longer period of time.
I have been a 24/7 carer of my son in a foreign country with no family or friends to help and it really is very hard work and extremely draining. I did not do it out of choice and to be honest, I would never recommend anyone do it out of choice, as you literally have NO time to yourself. After a while I managed to build up some friends who would help me out when required but sometimes I think it's easier for women to mix with other women and ask for help rather than men mix with other women or indeed other men (very rare I find - afterall, there are few father and baby groups around but plenty of mother and baby groups!).
Be prepared for some very hard times ahead with your lad as I think he will possibly react like a bereaved child for some time, especially if his family in Thailand will be having him full on for three months whilst you're in the UK and then he has to leave them. The first few weeks/month may seem like a holiday and go smoothly, but when he realises he's never going 'home' it could be pretty hard for him. You will have to be extremely patient and attentive, even when you are tired and drained or ill yourself. You will be leaving work every day and having your son's company EVERY night - no socialising with new colleagues or being able to get out and about and find friends of your own. From Friday evening to Monday morning for the next SIX YEARS or so you are unlikely to get much time for yourself, to pursue a new relationship or hobby or sport. I was going to suggest you join his school PTA in order to get to know other parents who may offer to help you out, but then you'd have to find a babysitter in order to go to the PTA meetings etc as it's adults only.
I am still a 24/7 parent, not in a relationship, but I do have family I can rely on and count on when my son is ill, they will babysit for me once a week so I can go out and as I work part time I am not rushed off my feet in a full time job so I do get some 'me' time and time to do the housework, shopping, washing and ironing when my son is at school and this does free up my time with him at the weekend. If I worked full time and then had to spend my weekends catching up on the housework, gardening, washing, ironing, shopping, taking my son to football matches without anyone else in the network I think I'd crack under the strain.
Do you realise what a mammoth task you are actually taking on? Is this job in the UK actually worth it?
Would you be able to do a trial run in Thailand for a whole month and look after your son or arrange his care without the help of family and friends and purely rely on paid professional services for the childcare etc and see how it goes? Spend your whole weekends with him and no one else you know, so go out to places and do new things, meet new people, then go home shattered after all this and repeat on Sundays, then go home and get ready for a full weeks work, making sure you finish on time to collect him before the childcare club closes (remember you will have no one to fall back on to ask them to get him if you're running late), get home, cook tea, go through his homework, etc etc. See how he reacts to not seeing family - just speaking to them on the phone/skype? This will at least give you a taster of what you're letting yourself in for for many years to come!