Just a few points from your OP.
'That way of he's too upset I can stop than having a tutor continue grilling him.'
That sounds more like a scenario from Local Authority standard intervention involving ignorant staff who assume difficult behaviours are due to naughtiness. This doesn't happen on ABA programmes, and if you ever see it you must show that tutor the door.
'NHS have suggested intensive interaction & PECS which we just started & he seems to be responding'
There is no evidence base for intensive interaction and PECS is ABA (though unlikely to be being done properly in most cases).
'I have been looking into ABA- they quote crazy costs plus am not sure wether I want him drilled for so 30-40 hours a week by a tutor.'
ABA isn't an organisation. 'They' can't quote any costs any more than Chemistry can quote costs. ABA is a science, - the science of learning. You analyse skills that need learning, and then teach them to the child according to their individual needs and motivations, acknowledging that cooperation and interest having the biggest influence of progress and pace (far more than drilling boring stuff) and so therefore children on ABA programmes are enthusiastic about what they are doing and proud of their achievements.
Also, like anything, the more you practice something, the better you get. 2 hours of violin practice a week could easily make you a violin player. 40 hours a week will do it faster. My son did between 10-15 hours of ABA a week and most of it was delivered by me. The most significant aspect of our programme was that as parents we learned how to slip learning target practice into ds' daily living without him noticing he was doing anything other than what made him happy.
If you PM me your email address I will send you a link to a video of my child's ABA programme. You can judge for yourself his level of happiness.