In a general sense it is clear that your son displays some developmental delays in a number of areas. While that might be instrumental in his incomplete toilet training, at his age it is best to rule out any physical explanation. A pediatric gastroenterologist would be an appropriate physician with whom to consult. It never hurts to be patient and supportive when it comes to toilet training, and if your son were younger I would not suggest any specialized evaluation. It is his chronological age that suggests an evaluation might be in order. However, it's important to recognize that, due to his developmental status, he is in most areas like a younger child.
Autism is one condition along a spectrum of conditions called Pervasive Developmental Disorders. Within each condition there is also a spectrum of severity. Some children may display mild forms of autism. Some children display some 'autistic-like' traits, but do not have the degree of dysfuction in socialization and communication that would be true of autistic children. Children with Asperger's Disorder would be examples. The diagnosis is dtermined by careful clinical assessment of the child's developmental status and symptoms, and is best done in a multi-discipline setting. Whatever genetic vulnerability he possesses is certainly exacerbated by his early experiences.
It would be prudent to have your son evaluated by a team of professionals, including developmental pediatrician, developmental psychologist, child psychiatrist, speech & language therapist and occupational therapist (sounds like you're already all set re: the latter two disciplines). It could well be that he displays at least some neurological immaturity; you already know that he displays other neurointegrative conditons (e.g., sensory integration problems, ADHD).
Your child's history raises several issues and concerns.
If your son was born only 6 weeks prematurely, it is unlikely the cause for his various developmental delays. Most 34-week gestation babies do well.
Children who have been insitutionalized for their first 3 years typically manifest many delays, though delayed walking is atypical even for this population. Although it sounds like he is making gains in many different developmental domains, it is quite possible that your child is suffering from more than "post-institutionalized child" syndrome.
It is very difficult to delineate between autism and "postinstitutionalized child syndrome" since both groups can exhibit lanaguage delays, sensory integration issues, and deficits in social interaction. The history of delayed language, flapping, sensory issues, and poor eye contact do indeed suggest an "autism spectrum disorder" or "pervasive developmental disorder" (PDD) of an uncertain cause. As for treatment services, your child would likely do well being placed in a special education class for high-functioning autistic children, though review of recent testing by a professional would be required to verify this. It is possible that an inclusion class may be adequate if functioning is high enough. Althoug grade retention is seldom recommended, it might be very prudent for your son if it would separate the 2 boys grade-wise and give the delayed one more time to master English and to develop more normal social skills.
The insitutionalization is the likely "smoking gun" causatively, but it is quite possible that other factors are involved as well. Although you could see a child psychiatrist or child neurologist for the autism evaluation, you may be best off seeing a developmental pediatrician -- preferably one with experience with Easter European adoptions. Consideration should be given to a limited medical work-up for the developmental delays. A DNA test for Fragile X and a routine chromosomal karyotype would be recommended, and you can discuss these with your pediatrician or consulting developmental pediatrician.
The delays in toileting and motor skills raise the issue of overall intellect or cognitive functioning. Your child's IQ has likely been estimated as part of previous developmental evaluations. This can be difficult to assess in children like this, since language is impaired and there can be large disparities between verbal and non-verbal skills.