Agree with the comments above, acting 'drunk' is typical of having a hypo. Its important to both test and treat it as quickly as possible.
It also sounds like he needs to increase his intake of food, why has he changed his diet ? Eating 3 small meals a day isn't going to sustain him if he's a large chap so increasing food intake is also important for sustaining his energy levels, just ensure he's carb counting correctly and recording insulin doses too, early on in diagnosis is important for analysing every detail as his insulin needs could change and his team need to be aware to make accurate dose adjustments if required.