The seven GCSE equivalent used to be the diploma, not the BTEC, although the full time level 2 extended certificate was worth 7 - it was usually taken post 16. I am not sure, but that might still be available.
I would agree with everything SecretSquirrels said - I am a curriculum designer and teacher and BTECs are not well regarded. Most BTECs started this year are worth the "same" as GCSEs for the guided learning hours they take (ie how many option blocks it occupied in your school's curriculum model, so a 3 hour a week course is 1 GCSE, 6 hours a week - roughly - is 2, etc), so if it took up one option choice it is worth one, and if two, it is worth two. There are no magic 7 GCSE fixes available pre-16 any more. However, there are some hangover ones so if your son is going into Y11 it might be worth double - so a two-option-block subject is worth 4.
BTECs are available at level 1 (grades G-D) and 2 (C-A*) but employers are sceptical, especially as Mr Gove has been slagging BTECs off for a while now.
Best bet is to call the curriculum team leader for Engineering or Technology at your son's school and ask for chapter and verse - get them to tell you which board it is and which qualification (diploma, certificate) etc it is. If you repost that here, I will tell you the GCSE equivalent.