What happens when the auto-entrepreneur turnover limit is exceeded?

My husband has received a letter saying he has exceeded the auto-entrepreneur business turnover limit in France. A few questions and some background so that we can clarify the business situation:

1. He set up an auto-entrepreneur business in France a last June. He remained under the 32,900 euros limit (circa 29,000 euros) for 2015. He only intends to work six months of the year. However URSSAF have sent a letter averaging his income out over the 12 months, and saying he should have stayed under 22,084 euro (32,900 x 245 / 365). Is there any recourse to stay as an auto-entrepreneur if he explains that his work is structured in six-month brackets and he intends to remain under 32,900 euros each year?
2. In the same letter, URSSAF say that he has ceased to be an (Regime micro social simplifie) and is now a profession independent. They have sent him a calendrier de vos pavements for 2016 cotisations, based on the 2015 revenue. Is this all he will be expected to pay or will there be additional factures for pensions, health or other elements of the system?
3. We have always been told that once you exceed this auto-entrepreneur limit you must go up to a SARL or EURL. However the URSSAF letter seems to indicate his can continue working for himself as a profession independent with these additional cotisation payments. Is this the case?
4. If so, is there a new limit on how much he can earn as a profession independent? If not, why does everyone say you must become a SARL after 32,900 euros if it is possible to effectively continue as self-employed under the profession independent classification? For his income it is not worth us setting up a SARL but this profession independent status (add the cotisations) seems very reasonable if he is able to continue.